This has happened on occasion and I have to restart mail. Hard to reproduce because most often control-command-G works as expected.
I checked SpamSieve Log.log and all I see is the following
no indication of why spam sieve crashed and why it took mail client down as well.
Launched: 2.9.47 (2947060) on macOS 10.15.7
Date: 2022-01-10 09:03:31 -0500 (EST)