The last few days/weeks I've been in contact with Peter and he consulted the ESET dev team to solve my issue. We tried several things, I sent several logs and tried a patched version of the filtering part but it didn't help. Finally my issue disappeared after I removed some certificates from the 'Manage Certificates' config box in Thunderbird. This were certificates I (long ago) accepted due to self-signed certificates or test servers. Some servers I connect to now (which have now a valid certificate) had an certificate in that box (but not all). It looks like somehow something got confused by those certificates.
After deleting my own accepted certificates, I could enable the IMAPS check in ESET again and everything works fine. It doesn't explain where and why things went wrong and unfortunately I cannot reproduce it any more. You could try this too (and make a backup of the certificates first for debugging purposes).