The htmltidy plugin as in the Backports.org version 2.32.3~bpo40+1 of ikiwiki does not play well with other usages of HTML Tidy since it has no possibility to use an alternative config file.
E.g. since I usually use HTML Tidy manually only to check and not to fix HTML, I have "markup: no" in my $HOME/.tidyrc which throws an awful lot of Perl warnings and renders all ikiwiki pages empty as soon as I enable htmltidy.
>> Hmmm, ok. Well, it didn't work. Empty pages, Perl Warnings. I moved my $HOME/.tidyrc away and it worked again. Had a short look into it and the only obvious non-default setting I found was write-back. I'll check what exactly caused the breakage and let you know. --[[XTaran]]
>>> Ok, found it. It indeed wasn't `write-back`, but `markup: no`. (I usually only want to see warnings and errors, not the fixed markup.) I now've corrected this in the bug report above. --[[XTaran]]