Yhyjyk

I have a 40 page long document that works fine for the most part, but starts having heaps of errors on different lines by the end. The document has a total of 42 errors, happening on several lines, and one line usually has 10-15 errors. The document compiles fine, and it looks as it's supposed to, but it still bothers me to have the errors, and why I have them, so help would be appreciated.

I simplified the document to share here, as requested by comments, and now it looks like this:

documentclass[a4paper,11pt,pdftex]article

usepackage[utf8]inputenc
usepackage[magyar]babel %needed in Hungarian reports
usepackagefancyhdr %for the nice header
usepackagegraphicx %grapics input
usepackagetikz %tikz figures
usepackage[colorinlistoftodos]todonotes
renewcommandbaselinestretch1.5

usepackageamsmath
usepackageamsthm
usepackageamssymb
usepackagebraket
usepackagecaption
usepackagesubcaption
usepackage[colorinlistoftodos]todonotes % szövegbeli kommentekhez, szövegdobozokhoz
usepackagecomment %többsoros kommentekhez
usepackagetextgreek %görög betűk math mode-on kívül
usepackagefixltx2e % felső és alsó indexekhez
usepackagesiunitx %szép celsius jelért
usepackaget1enc %mert kéri...
%usepackage[justification=centering]caption %hogy a képaláírások középen legyenek
usepackagearray % hogy a táblázat oszlopainak szélességét állítsam
usepackagetabu % ez meg auomatikusan kitölti a táblázatot oldalszélesre

usepackage[a4paper,left=2.5cm,right=2.5cm,top=2.5cm,bottom=2.5cm,pdftex]geometry %margins

usepackagecolor
usepackagexcolor
usepackageurl
usepackagehyperref %links
hypersetup
colorlinks=true,
linkcolor=blue,
urlcolor=blue,
citecolor=red,
unicode=true


begindocument
pagestylefancy

sectionResultslabelresults

clearpage

enddocument

This version has 13 errors, all on line 47 (the clearpage line). Here are the errors I'm getting:

! Undefined control sequence.
bbl@fixname #1->begingroup def bbl@tempe 
 l@edef bbl@tempd noexpand...
l.47 clearpage

The control sequence at the end of the top line
of your error message was never def'ed. If you have
misspelled it (e.g., `hobx'), type `I' and the correct
spelling (e.g., `Ihbox'). Otherwise just continue,
and I'll forget about whatever was undefined.

! Argument of bbl@tempd has an extra .
<inserted text> 
 par 
l.47 clearpage

I've run across a `}' that doesn't seem to match anything.
For example, `defa#1...' and `a}' would produce
this error. If you simply proceed now, the `par' that
I've just inserted will cause me to report a runaway
argument that might be the root of the problem. But if
your `}' was spurious, just type `2' and it will go away.

Runaway argument?
noexpand @ifundefined noexpand bbl@tempe languagename bbl@tempd ETC.
! Paragraph ended before bbl@tempd was complete.
<to be read again> 
 par 
l.47 clearpage

I suspect you've forgotten a `}', causing me to apply this
control sequence to too much text. How can we recover?
My plan is to forget the whole thing and hope for the best.

! Use of begingroup doesn't match its definition.
<to be read again> ...MAGYAR1begingroup def 
 L@edef par 
l.47 clearpage

If you say, e.g., `defa1...', then you must always
put `1' after `a', since control sequence names are
made up of letters only. The macro here has not been
followed by the required stuff, so I'm ignoring it.

! Missing inserted.
MakeUppercase ... uppercase #1reserved@a 

l.47 clearpage

Where was the left brace? You said something like `defa}',
which I'm going to interpret as `defa'.

! Missing } inserted.
<inserted text> 
 }
l.47 clearpage

I've inserted something that you may have forgotten.
(See the <inserted text> above.)
With luck, this will get me unwedged. But if you
really didn't forget anything, try typing `2' now; then
my insertion and my current dilemma will both disappear.

! Extra }, or forgotten endgroup.
@begin@tempboxa ...begingroup #2color@endgroup }
 def width wd @tempbox...
l.47 clearpage

I've deleted a group-closing symbol because it seems to be
spurious, as in `$x$'. But perhaps the } is legitimate and
you forgot something else, as in `hbox$x'. In such cases
the way to recover is to insert both the forgotten and the
deleted material, e.g., by typing `I$}'.

! Extra }, or forgotten endgroup.
<argument> ...adwidth }raggedleft f@nch@orh }}
 headrule 
l.47 clearpage

I've deleted a group-closing symbol because it seems to be
spurious, as in `$x}$'. But perhaps the } is legitimate and
you forgot something else, as in `hbox$x'. In such cases
the way to recover is to insert both the forgotten and the
deleted material, e.g., by typing `I$}'.

! You can't use `hrule' here except with leaders.
headrule ...width plainheadrulewidth fi hrule 
 @height headrulewidth @...
l.47 clearpage

To put a horizontal rule in an hbox or an alignment,
you should use leaders or hrulefill (see The TeXbook).

! Missing number, treated as zero.
<to be read again> 
 vskip 
l.47 clearpage

A number should have been here; I inserted `0'.
(If you can't figure out why I needed to see a number,
look up `weird error' in the index to The TeXbook.)

! Illegal unit of measure (pt inserted).
<to be read again> 
 vskip 
l.47 clearpage

Dimensions can be in units of em, ex, in, pt, pc,
cm, mm, dd, cc, nd, nc, bp, or sp; but yours is a new one!
I'll assume that you meant to say pt, for printer's points.
To recover gracefully from this error, it's best to
delete the erroneous units; e.g., type `2' to delete
two letters. (See Chapter 27 of The TeXbook.)

! Missing } inserted.
<inserted text> 
 }
l.47 clearpage

I've inserted something that you may have forgotten.
(See the <inserted text> above.)
With luck, this will get me unwedged. But if you
really didn't forget anything, try typing `2' now; then
my insertion and my current dilemma will both disappear.

! Missing endgroup inserted.
<inserted text> 
 endgroup 
l.47 clearpage

I've inserted something that you may have forgotten.
(See the <inserted text> above.)
With luck, this will get me unwedged. But if you
really didn't forget anything, try typing `2' now; then
my insertion and my current dilemma will both disappear.

! Missing } inserted.
<inserted text> 
 }
l.47 clearpage

I've inserted something that you may have forgotten.
(See the <inserted text> above.)
With luck, this will get me unwedged. But if you
really didn't forget anything, try typing `2' now; then
my insertion and my current dilemma will both disappear.

! Missing } inserted.
<inserted text> 
 }
l.47 clearpage

I've inserted something that you may have forgotten.
(See the <inserted text> above.)
With luck, this will get me unwedged. But if you
really didn't forget anything, try typing `2' now; then
my insertion and my current dilemma will both disappear.

! Extra }, or forgotten endgroup.
@outputpage ...or hb@xt@ textwidth @thehead 
 color@endbox }dp @tempb...
l.47 clearpage

I've deleted a group-closing symbol because it seems to be
spurious, as in `$x}$'. But perhaps the } is legitimate and
you forgot something else, as in `hbox$x'. In such cases
the way to recover is to insert both the forgotten and the
deleted material, e.g., by typing `I$}'.

Not sure if it matters, but I'm writing in an online editor (overleaf).

You should load fancyhdr before babel when the option magyar is used (as directed by a warning issued by the language definition file). However, this conflicts somehow with MakeUppercase.

Either you redefine sectionmark to avoid it, or load textcase with the overload option.

documentclass[a4paper,11pt]article

usepackage[T1]fontenc
usepackage[utf8]inputenc
usepackagefancyhdr %for the nice header
usepackage[magyar]babel %needed in Hungarian reports
usepackagegraphicx %grapics input
usepackagetikz %tikz figures
usepackage[colorinlistoftodos]todonotes

usepackageamsmath
usepackageamsthm
usepackageamssymb
usepackagebraket
usepackagecaption
usepackagesubcaption
usepackage[colorinlistoftodos]todonotes % szövegbeli kommentekhez, szövegdobozokhoz
usepackagecomment %többsoros kommentekhez
usepackagetextgreek %görög betűk math mode-on kívül
%usepackagefixltx2e % felső és alsó indexekhez
usepackagesiunitx %szép celsius jelért
%usepackaget1enc %mert kéri...
usepackage[justification=centering]caption %hogy a képaláírások középen legyenek
usepackagearray % hogy a táblázat oszlopainak szélességét állítsam
usepackagetabu % ez meg auomatikusan kitölti a táblázatot oldalszélesre
usepackage[overload]textcase

usepackage[
 a4paper,
 left=2.5cm,
 right=2.5cm,
 top=2.5cm,
 bottom=2.5cm,
 headheight=13.6pt,
]geometry %margins

%usepackagecolor
usepackagexcolor
%usepackageurl
usepackagehyperref %links
hypersetup
 colorlinks=true,
 linkcolor=blue,
 urlcolor=blue,
 citecolor=red,
 unicode=true


renewcommandbaselinestretch1.5
pagestylefancy

begindocument

sectionResultslabelresults

clearpage

enddocument

I also made a few changes: I removed the option pdftex everywhere, which should not generally be used; instead of t1enc it's better to load fontenc; loading color is useless if xcolor is used; fixltx2e is included in the latest releases of LaTeX; url is loaded by hyperref.

Finally, I added a setting to headheight, as directed by fancyhdr.