All the PCs are running Win 10 x64 1809.
Just after enabling advanced logging, NOD32 has notified an update. I have done it and restarted the system and now all the IMAPs servers work fine again. 😦
It's strange because the first thing I did, after experiencing the issue, was to check updates, but the software appeared to be up-to-date.
Thank you very much. I hope the problem will not occur again.