I have now (see last post) found out that the main HD of the server is failing. The hosting company will replace both HDs (main & backup). While I am pretty sure that the second HD is fine, I had to rent a FTP server on their farm to make an additional backup.
I am currently copying all needed data to the backup FTP, then I will ask the hoster to put new HDs into the system, then will have to setup everything again and copy over the data from the backup. Normally the re-setup of the server will take 5 hours, I will most likely need another couple of hours to setup everything again. If the old HD is not too broken, there is a chance that this goes quickly. We will see.
Since it’s getting late here, I will simply let the backup run overnight while I sleep and then try to do the rest tomorrow. Since I never did that before and since I do not know if I will be very busy tomorrow with other stuff and when the hoster actually will be able to replace the HDs, there is not ETA for this yet. The server will be down sometime (including the website), and then hopefully be back up again not too much later. Please check the facebook page for updates and IRC to chat.