Yes, as I said - if you are using the Eset Security (or antivirus), just set the IMAP setting of the Eset to the default as you can see in the picture.
I had to untick the “Enable IMAP checking” in ESET in order to receive any Gmail, realising that googlemail documents would not be scanned. When I thought the problem had been fixed by an update I checked the box and it still seemed to work. I do get the occasional eM error messages but not sure why.
I’ve used Eset Smart Security for years, but at some point, it prevented me from downloading attachments in e-mails correctly in Thunderbird: those attachments were corrupted.
I’ve switched to Bitdefender and have had no issues since then (‘knock on wood’… 
I don’t remember… May be I’ll find notes on my home computer when I’ll be back from holidays
We know about issues with Avast and AVG, ESET is quite new to us. Anyway usually removing IMAP from being scanned or altering security policy in Account management form IMAP to “do not use secure connection” helps.
Jan