In Koha 21.11.12, when having the localcoverimages preference active and uploading a cover page of a record, it saves it correctly but in the normal view it marks error 500 and does not allow to consult the information, the MARC, tagged MARC and items view works correctly. If the localcoverimages preference is deactivated, the record is visible in the normal view, but without its cover page.
I can reproduce this error in 21.11.12 too: Template process failed: undef error - The method Koha::CoverImage->count is not covered by tests!
This also seems to exist in 21.11.11
This is fixed in 21.11.13 by Bug 30976.
*** This bug has been marked as a duplicate of bug 30976 ***