Some updates.
I've been fiddling around with this during the weekend but haven't made 
much progress. Removing all modules, in case they were causing problems, 
didn't help.
I have, however, pinpointed where in the kernel it hangs. It stops while 
writing the SLP_EN bit. I guess that's where it should actually go into 
sleep mode. The bit is set in two registers but the hang is when it 
writes the first one. Changing the order of them still causes the first 
one to hang (B in that case). I also tried combining SLP_TYPx and SLP_EN 
but again, hang during the first write.
I'm out of ideas now so I *really* need some tips on what to try next. 
There is nothing more to debug around the crash-point so the problem 
must be somewhere during the preparation.

Rgds
Pierre