Remember to cover the basics, that is, what you expected to happen and
what in fact did happen.

Be sure to consult the FAQ section in the manual before submitting
a bug report.  In addition check if the bug is reproducable with an
up-to-date version of AUCTeX.  So please upgrade to the version
available from http://www.gnu.org/software/auctex/ if your
installation is older than the one available from the web site.

If the bug is triggered by a specific (La)TeX file, you should try
to produce a minimal sample file showing the problem and include it
in your report.

Your report will be posted for the auctex package at the GNU bug
tracker.  Visit http://debbugs.gnu.org/cgi/pkgreport.cgi?pkg=auctex
to browse existing AUCTeX bugs.
------------------------------------------------------------------------

The LaTeX package doc, that is usually used via class ltxdoc for the
documentation part of dtx-files, declares ^^A as comment character.
But auctex's doctex mode fails scanning for balanced {...} pairs, if
} is immediately followed by ^^A. You can test it, with this example file:

--- test.dtx ---
% \begin{macro}{\test}^^A This is an official dtx-comment
% This does not work. Try to double-click to the closing brace before the
% comment and you will get: |Scan error: "Unbalanced parantheses", 22, 22|
% \end{macro}
%
% \begin{macro}{\test}%^^A This is a workaround for an official dtx-comment
% This does work, but the \% after the closing brace is not wanted.
% \end{macro}
% \endinput
% Local Variables:
% mode: doctex
% TeX-master: t
% End:
--- end of file ---

Emacs  : GNU Emacs 26.1 (build 1, x86_64-suse-linux-gnu, GTK+ Version 3.24.4)
Package: 12.1

current state:
==============
(setq
 AUCTeX-date "2017-12-10"
 window-system 'x
 LaTeX-version "2e"
 TeX-style-path '("/usr/share/emacs/site-lisp/auctex/auto"
		  "/usr/share/emacs/site-lisp/auctex/style"
		  "/home/mjk/.emacs.d/auctex/auto"
		  "/home/mjk/.emacs.d/auctex/style" "auto" "style")
 TeX-auto-save t
 TeX-parse-self t
 TeX-master t
 TeX-command-list '(("biber" "biber %s.bcf" TeX-run-BibTeX nil t)
		    ("TeX" "%(PDF)%(tex) %`%S%(PDFout)%(mode)%' %t"
		     TeX-run-TeX nil
		     (plain-tex-mode texinfo-mode ams-tex-mode) :help
		     "Run plain TeX")
		    ("LaTeX" "%`%l%(mode)%' %t" TeX-run-TeX nil
		     (latex-mode doctex-mode) :help "Run LaTeX")
		    ("Makeinfo" "makeinfo %t" TeX-run-compile nil
		     (texinfo-mode) :help "Run Makeinfo with Info output")
		    ("Makeinfo HTML" "makeinfo --html %t" TeX-run-compile
nil
		     (texinfo-mode) :help "Run Makeinfo with HTML output")
		    ("AmSTeX" "%(PDF)amstex %`%S%(PDFout)%(mode)%' %t"
		     TeX-run-TeX nil (ams-tex-mode) :help "Run AMSTeX")
		    ("ConTeXt" "texexec --once --texutil %(execopts)%t"
		     TeX-run-TeX nil (context-mode) :help "Run ConTeXt
once")
		    ("ConTeXt Full" "texexec %(execopts)%t" TeX-run-TeX
nil
		     (context-mode) :help "Run ConTeXt until completion")
		    ("BibTeX" "bibtex %s" TeX-run-BibTeX nil t :help
		     "Run BibTeX")
		    ("View" "%V" TeX-run-discard-or-function t t :help
		     "Run Viewer")
		    ("Print" "%p" TeX-run-command t t :help "Print the
file")
		    ("Queue" "%q" TeX-run-background nil t :help
		     "View the printer queue" :visible TeX-queue-command)
		    ("File" "%(o?)dvips %d -o %f " TeX-run-command t t
:help
		     "Generate PostScript file")
		    ("Index" "makeindex %s" TeX-run-command nil t :help
		     "Create index file")
		    ("Check" "lacheck %s" TeX-run-compile nil (latex-mode)
		     :help "Check LaTeX file for correctness")
		    ("Spell" "(TeX-ispell-document \"\")" TeX-run-function
nil
		     t :help "Spell-check the document")