Thanks for your suggestion. Both DEBUG and LOG are enabled, and I'm getting a lot of these msgs:
[20-Jun-2017 13:01:07 Europe/Zurich] PHP Notice: Undefined index: pk_i_id in /home/[...]/oc-includes/osclass/frm/Category.form.class.php on line 33
But nothing else, no fatal errors.
I'm running a few plugins, but disabling them makes no difference at all. No mods whatsoever to the core code.