Matthias fixed this earlier in https://gerrit.wikimedia.org/r/#/c/197026/ , but it needs to be cherry-picked to 1.25wmf21. It is currently affecting everything except MediaWiki.org and the test wikis (this means it affects all Wikipedias).
Amir hit this issue in T93492: Fatal in getUserLinks for a revision that undoes a suppression, though I don't think that led to the other bug (editing the header does not trigger a visible diff).