That's because a normal Exclusive board has a built-in voltage regulator, which the Muenchen S does not have. 9 V are too much for it; many Risc-modules (also without regulator) have been accidentally roasted with that voltage. They were delivered with a special 5 Volt adaptor, the HGN5050.

I guess, 5,5 or max. 6 V could work...

best regards,
Robert