The IMAP sensor has a problem. When trying to log in, the following response is returned by the server:
Command requires 2 parameters
I've discovered the problem is because of the password. It needs to be surrounded by quotes (because it contains special characters), which the sensor is not doing. I tested this by using the same email address and password that PRTG is trying to use, without quotes, and got the same error message. I then attempted the login and surrounded the password with quotes, and it worked.