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).
Marco5342 received kudos from Peter Randziak in Protocol filtering stops access to emails
Marco5342 received kudos from JeffJ in Protocol filtering stops access to emails