This is possibly a HHVM issue (I am running nightly version since the latest release 3.11.0 is fatally broken).
Triggered by URLS such as w/i.php?title=Project:About/da&diff=6454985&oldid=4647374 or /w/api.php?hidebots=1&days=7&limit=50&action=feedrecentchanges&feedformat=atom.
Given there are no line numbers or file names, this might be related to assertions in DairikiDiff.php using $this:
459: assert( '$y < $this->seq[$k]' ); 510: assert( '$ypos != $this->seq[$end]' );
See also https://github.com/facebook/hhvm/issues/5128 and T116266: Undefined property: DiffEngine::$seq.