Fixed.
For the record: to fix this, a sysadmin needs to find out the offending
page (probably a comment page) by looking at webapps/docs/data/edit-log
and remove the offending changelog entry there and also in the changelog
of the page in webapps/docs/data/pages/<name of the page>/edit-log.
I won't say what exactly causes this moinmoin to break down here,
because I don't have an immediate fix for it atm.
Chris