It seems our provider for the server is not able to keep it running. In the past days the server was either not reachable by everyone or even completly offline. We guess it was only reachable by some users because of routing problems or hardware issues in the provider's network. The trace route results have shown us time outs behind one or more gateways of the provider.
Because of the continuing downtimes we think about other solutions.
The server experienced an unexpected downtime for ~7 hours today, reason unknown. A problem with software and hardware can be precluded, seems there were some problems with the wires at the datacenter.
Debian Squeeze was released today and we've upgraded BoxBox.org. Everything running flawlessly and software is more up-to-date.