The system drive of our Mac mini server crashed and had to be replaced with a new hard disk
Thanks to iFixIt, the repair went well. Unfortunately, the system drive turned out to be the one in the upper bay (i.e. the one that is mounted instead of the super drive, below the second hard disk) and I did not find a guide for replacing that one.
After removing the logic board and the power supply completely from the aluminum body, the upper hard disk bay was accessible (unfortunately, we did not take any pictures during the repair).
Since the Mac mini server has two hard drives, it might be useful to identify the individual drives with System Profiler before opening the Mac mini...
After restoring the system from a time machine backup, our wiki server did not work. The solution to this problem was quite simple (as described in several support forums). We only had to create the following directory:
sudo mkdir /Library/Logs/wikid
sudo chown teamsserver:teamsserver /Library/Logs/wikid