That page, and many others, links to another page with just a plain name:
This is read by man2html and passed to MANProtocol::readManPage() with a filename of "../PAM.8". This is then resolved against the containing directory "/usr/share/man/man8", becoming "/usr/share/man/PAM.8". No such file exists and the mandir.entryList().first() asserts because the list is empty.
The relative filename is correct in the case of other pages. e.g. telinit(8) which uses:
So it is not enough to simply ignore the "..". The change to man2html here attempts to correct the reference by detecting whether it is of the form "../page.sect" and is not "../manN/page.sect", and removing the ".." component only in the first case.
While investigating this I found it useful to have a better diagnostic if the .so page could not be found. The change here to kio_man.cpp detects that case and displays an error page instead of asserting. This should only happen if there is a bug in the man page, but it is useful to have a pointer so that the upstream man page can be fixed.