Some tips for large log file:

• Since it's really hard to look for (file and ) in that case, you can use some LaTeX hook to log some message when the file changes:

  \ExplSyntaxOn
  \seq_new:N \__file_list
  \seq_put_right:Nn \__file_list {\msg_expandable_error:nn{}{this~cannot~happen!}}
  \seq_put_right:Nn \__file_list {<main~file>}
  \AddToHook{file/before}{
      \seq_put_right:NV \__file_list \CurrentFile
      \wlog {enter~file\use_none:n{}::\CurrentFile}
  }
  \AddToHook{file/after}{
      \seq_pop_right:NN \__file_list \__last_file
      \seq_get_right:NN \__file_list \__last_file
      \wlog {return~to~file\use_none:n{}::\__last_file}
  }
  \ExplSyntaxOff
  

  (obviously the effect is, you get a line enter file::X and return to file::X in the log. Search for file::. The \use_none:n {} is to prevent the message from being found while doing macro expansion, if macro logging is enabled.)

• A "binary search" alternative is to insert a (commented) empty line to some TeX files, then see if the line number increases. (it's "brute force", but works most of the time, except if catcode is weird.)

• Note that, even if some particular \if is reported as the culprit, it's not necessarily the cause. For example

  \if ... % A
  
      \if ... % B (accidentally inserted)
  
  \fi % C
  

  Clearly the one at B is at fault here, nevertheless TeX will report A is incomplete. And A might be deep buried inside some library code (e.g. if you have incomplete \if within some .sty file this can happen)

• Generally speaking, it's useful to search for if.* <line number>} (regex) from the end of the file, since you can observe that if there are multiple such \if, the last one will be likely at fault; nevertheless if the first hit is a \fi then skip through one \if.

  Example log

  {\if entered on line X}
  {\else: \if entered on line X}
  {\fi: \if entered on line X}
  
  {\if entered on line X}
  {\else: \if entered on line X}
  {\fi: \if entered on line X}
  
  {\if entered on line X}
  {\else: \if entered on line X}
  {\fi: \if entered on line X}
  
  {\if entered on line X}
  
  {\if entered on line X}
  {\else: \if entered on line X}
  {\fi: \if entered on line X}
  

  In this example it can be seen that if you search for the log lines from the end backwards, you'll first see a \fi, then skip one \if, then find the offending command immediately. (there's (level X) to partially help with search for matching lines, but no more than that.)