Here's the missing screenshot ;)

Problem solved.
the post types have been deselected in the config.
I don't know if this is related to the upgrade or just a coincidence, but after having activated the corresponding types all works fine again.

Please check the attached screens and please make screenshots of how they were configured before you start changing the setting so we can track down what is missing in the auto-upgrade process.

Ah you've been too fast ;) Do you remember which posts types were disabled which should have been enabled?

6.6.3 Had a similar problem using a standard blog, with bootstrap forum skin, but can't remember if it was the post(1) or forum topic(200) that was disabled.