There's an open jam session on the first Thursday of every month here in Tübingen. They inexplicably call themselves "OpenSession - D))))onnerstHaag" and advertize only on Facebook with Dänglisch:

Every first thursday we meet to play jam-music
no limits of style and genre. Every level.
Free admission, nice folks, perhaps you too....
jazzhippyfolkypoppy,

Cafe Haag, Tübingen
20.30 -23.30

I probably won't be able to come this week, but I'll try to make it in November and report back.