I have three email accounts and with only one of them I get the following message, not always, but quite often. All three have IMAP settings and I am certain that the password is not the problem. My email client for all three accounts is Thunderbird version 52.6.0 but it only effects one account, the problem is also on my Samsung S6 phone.
It has been suggested that this may be a problem with my Anti Virus app.