maintengine.py has trouble with TypeError

Sometime this last week our maintengine has stopped working:

/#/usr/local/nav/bin/maintengine.py Traceback (most recent call last): File "/usr/local/nav/bin/maintengine.py", line 45, in <module> main() File "/usr/local/nav/bin/maintengine.py", line 39, in main check_devices_on_maintenance() File "/usr/local/nav/lib/python/nav/maintengine.py", line 479, in check_devices_on_maintenance send_event(events, maxdate_boxes, boxes_off_maintenance) File "/usr/local/nav/lib/python/nav/maintengine.py", line 343, in send_event str(curr_event['maint_end']))) TypeError: not all arguments converted during string formatting

NAV 3.13.0

I guess something has gone wrong in the database. Where to look?

--Ingeborg