LHC@home

sft-nightlies.cern.ch isn't used very often.
Hence, it's unmounted after a while and remounted when you access /cvmfs/sft-nightlies.cern.ch.

Since sft-nightlies.cern.ch is not replicated to all stratum-1 cvmfs servers your client checks for the "closest" server following the server order given in the cvmfs configuration.

The messages you posted are not really errors if the following succeeds:
"cvmfs_config probe sft-nightlies.cern.ch" returns "OK"
then "cvmfs_config stat sft-nightlies.cern.ch" returns something like
VERSION PID UPTIME(M) MEM(K) REVISION EXPIRES(M) NOCATALOGS CACHEUSE(K) CACHEMAX(K) NOFDUSE NOFDMAX NOIOERR NOOPEN HITRATE(%) RX(K) SPEED(K/S) HOST PROXY ONLINE
2.9.1.0 38605 0 22772 81034 3 1 9906090 9961473 0 130560 0 0 0.000 4 20 http://s1bnl-cvmfs.openhtc.io/cvmfs/sft-nightlies.cern.ch http://192.168.1.3:3128 1

Nonetheless, remounting can take a short while.

It did it again tonight. Places applet/file manager and recycle bin/Trash applet both locked up solid. No errors in logs beside the same things. Only ever does it running this.

Those commands return the correct outputs now, but I did forget to run them while the applets were locked up.

I have no idea if this is relevant, but it was worth a try.
I noticed that on my two Mint rigs both were using the Bionic cern repositories rather than the Focal which the rest of the system, and my two Ubuntu Server headless rigs, use.
I have changed the settings so it is consistent, updated cvmfs and fuse2, and rebooted. I guess I'll see if that makes things better, worse, or has no effect at all.

Nope, it did it again.
Nothing in the error logs, just the file manager and trash applet locked up hard.
Filesystem access from the terminal is normal.
System has all updates installed.
Everything normal again after reboot.

Ok, an update.
I completely disabled the thumbnailer service in the system and the issue hasn't happened since.
At this stage, unless something changes, I suspect that was the problem. The thumbnail creation service was trying to handle the CVMFS directories and getting overloaded.