Abstract dumps on urwikibooks fails with MWUnknownContentModelException from ContentHandler.php on 5 pages, which all have content model flow-board when you look at the entry for the current revision in the content table. Flow was turned off (see T207627 now re-opened) but it seems the maintenance cleanup script didn't do everything it should have in this case. This is similar to T220594 or dewkiversity (with associated T20762 where Flow was disabled on that wiki).
Info from the database on the pages, revisions, slots and content can be found at T207627#5105044 and T207627#5105445
I ran the abstracts job manually with a hacked version of the code that skips over these errors, but it would be nice to get the entries cleaned up before the next run n the 20th.