[tor-relays] Relay stuck at Bootstrapped 0%

I am running two relays on debian (one on Raspian 9 and one on Debian 9) and one of them for over three months now. Some time ago the older one started to loop at startup. I reinstalled tor and it worked for some time only to loop again. The last notices entry looks like this "Feb 16 19:36:14.000 [notice] Bootstrapped 0%: Starting" and the next is "Feb 16 19:42:08.000 [notice] Tor 0.2.9.14 (git-a211f886ad759cab) opening log file." then it tells about opening the listeners and parsing the GEOIP files, then my relay identity and the bootstrapped 0% entry again (left some informational entries out). The debug log looks like it is getting descriptors and stuff correctly. What could be the problem?