It seems that my hoster Dreamhost ran into some problems yesterday. Lot's of websites returned 'Internal Service Errors' or ' Service Temporarily Unavailable'. These errors only occurred when addressing php content and not while accessing static html-files. So most likely a PHP (config) 'feature'.
The thing that annoys me the most about this downtime is not that it lasted almost a day, but the lack of communication. No mention of problems on their status page, and support-feedback was also thin.
I rather have a couple of days downtime, and knowing what the problem is (hard disk crash, power failure, human error), than 1 hour and not (ever) knowing what happened.
Anyway, the problem seems to be solved (for now), but I'm keeping my fingers crossed on the feedback issue.
UPDATE: Got feedback from Dreamhost.com that everything should be working. No feedback (yet) on the cause of the problem though.