You Squid service stops (nearly) nightly and has to be started manually in the morning.
You notice entries like this in
cache.log of Squid:
2020/10/01 02:24:38 kid1| diskHandleWrite: FD 14: disk write error: (28) No space left on device FATAL: Write failure -- check your disk space and cache.log Squid Cache (Version 3.5.23): Terminated abnormally.
Make sure you always have enough disk space available. Even temporary out-of-disk events will lead to Squid stop working.
There might be processes running in the night consuming disk space (whic will cause Squid to stop) and freeing the disk space afterwards (i.e. logrotate).