Martin Pohlack wrote:
> Sorry for the noise.
> 
> A local package globally modified outline-heading-end-regexp which broke
> folding via outline-mode.

It wasn't a local package after all: simple-wiki.el is to blame here.

It globally modifies "outline-regexp" and "outline-heading-end-regexp".

For those using simple-wiki.el, please find a patch attached (already
reported upstream).

Cheers,
Martin

PS.: This is probably the same problem as described in an earlier thread
called "Folding stops working".