Under Help >> User Files >> open the Logs folder >> open the folder named after todays date >> there will be a file called MWPapp_[Account Name]_.log that you can open to have a look at.
With IMAP, you're interested in the response after the LOGIN
e.g. This is sending a login that gets an invalid response.
12-18-2019 19:49:27.336 Info C: LOGIN *******
12-18-2019 19:49:30.644 Info IMAP SERVER LOGIN: mw2 NO Incorrect username or password.
If that turns up nothing obvious, you can try sending me the logs. To do that go Help >> Send Support Logs >> enter a brief note like 'as per forum' and in the Ticket Number field enter chris >> Send Logs >> reply back here once sent, so I can check they arrived OK.