POP3 polling: "Sorry an error has occured"

I have Discourse up and running and wish to enable POP3 polling. I have our office mail server and I checked that POP3 works with plain text on port 110 (unencrypted is ok because it is on private LAN)

There is now a message “Sorry, an error has occured” . How do I find out what is the error and what is causing it?