The MPS GrandOrgue Forum was down for about 3 days.
After an update of the forum software to SMF 2.0.14 the forum didn't load. It showed, that the needed PHP-version is actually 5.4 and not 5.3 as promised by the update-requirements.
After updating the server software, now we have the needed PHP version and so the forum now is online again.
Sorry for the interruption.