Yes, thanks, and this is a problem for the unstable/ URL too.

classGtk_1_1Entry.html does seem to be in the tarball, so I guess this is a problem with library.gnome.org. Reassigning.

I had a quick look, and a possible cause is those errors:

 $ xmllint classGtk_1_1Entry.html 
classGtk_1_1Entry.html:1149: parser error : CharRef: invalid decimal value
ion in the entry contents (returned by get_text()) to a position in the entry&#3
                                                                               ^
classGtk_1_1Entry.html:1149: parser error : xmlParseCharRef: invalid xmlChar value 0
ion in the entry contents (returned by get_text()) to a position in the entry&#3
                                                                               ^
classGtk_1_1Entry.html:1170: parser error : CharRef: invalid decimal value
ion in the entry contents (returned by get_text()) to a position in the entry&#3
                                                                               ^
classGtk_1_1Entry.html:1170: parser error : xmlParseCharRef: invalid xmlChar value 0
ion in the entry contents (returned by get_text()) to a position in the entry&#3
                                                                               ^
classGtk_1_1Entry.html:1204: parser error : CharRef: invalid decimal value
ion in the entry contents (returned by get_text()) to a position in the entry&#3
                                                                               ^
classGtk_1_1Entry.html:1204: parser error : xmlParseCharRef: invalid xmlChar value 0
ion in the entry contents (returned by get_text()) to a position in the entry&#3
                                                                               ^
I didn't find occurences of the broken "&#3..." in other files.

I guess it's complaining about that &#3 in this <a> code in the html:
  title="Converts from a position in the entry contents (returned by get_text()) to a position in the entry&#3..."

  Doxygen takes that from the comment for the function that it's linking to with that <a> link:

  /** Converts from a position in the entry contents (returned
   * by get_text()) to a position in the
   * entry's Pango::Layout (returned by get_layout(),
   * with text retrieved via pango_layout_get_text()).
   * @param layout_index Byte index into the entry layout text.
   * @return Byte index into the entry contents.
   */
  int layout_index_to_text_index(int layout_index) const;

I guess that it's meant to be a three-digit decimal number with a closing ;, but that doxygen is truncating and elipsizing it after escaping the ', instead of before. Reassigning to doxygen.

Unfortunately seen the time the issue was reported and the information available (no small example attached) it is hard to see what went wrong.
Looking at the still available page https://developer.gnome.org/gtkmm/stable/classGtk_1_1Entry.html the problem was not present here anymore.

I think the problem has been solved in the mean time.

As discussed in https://github.com/doxygen/doxygen/pull/734 , Doxygen has moved its issue tracking to 

   https://github.com/doxygen/doxygen/issues

All Doxygen tickets in GNOME Bugzilla have been migrated to Github. You can subscribe and participate in the new ticket in Github. You can find the corresponding Github ticket by searching for its Bugzilla ID (number) in Github.

Hence I am closing this GNOME Bugzilla ticket.
Please use the corresponding ticket in Github instead. Thanks a lot!