Hi Marcus, thanks for your reply.
is global tables turned on in your network settings?
Yes.
are you on the latest updates?
Yes.
does it work with other event lists and calendars?
What do you mean with other event lists / calendars?
do the events show up on your subsites (with and without WPFC)?
Yes. The events show up on the sites where they were posted. But not in the main blog, also not after the post rebuild.