If you are using ESET/NOD32 security/AV then probability is high that ESET is causing your problems. According to ESET forums, you should temporarily disable SSL on ESET until they solve the problem per https://support.eset.com/en/kb3126-disable-ssl-filtering-in-eset-windows-products?ref=esf - the problem is being reported by dozens of users https://forum.eset.com/topic/26517-problems-with-thunderbird-and-imaps/ https://forum.eset.com/discover/ You might also inform them of your difficulty.