Comment 26 for bug 1906152

An update.

I completely uninstalled removed calibre, rebooted and then installed the last version that I know worked (5.6), with no plugins.. The issue persisted (which does tend to show that the issue isn't with calibre, as you said.
I upgraded to Big Sur and rebooted - same issue.
I upgraded calibre 5.8.1 (on Big Sur) and rebooted - same issue.
I tested a lot of settings that may have had some effect, with no success.

I've found a workaround for the moment, by writing an AppleScript that restarts calibre if it ever closes, so it's not as urgent from my point of view now. I'm posting this to add to your data about this issue, just in case it's useful.

Now, if I can get the Stop Server button in Preference not to crash calibre completely... Start Server and Test Server work fine, but not Stop Server.