The error refers to the IMAP server but complains about POP not being enabled. What is the Server Type in Account Settings, Server Settings, and what is entered in Server Name?

Thanks but still no fix'

it looks as if this happened in December after update as incoming mail also stopped see attachments

You can't access a POP server with the IMAP server name. Change it to pop.gmail.com, but first enable POP in gmail webmail settings. Also, change the authentication to OAuth2, same for the outgoing smtp.

If you want an IMAP setup, you have to add the account as IMAP, not just change the settings.

http://kb.mozillazine.org/Convert_a_POP_account_to_a_IMAP_account

http://kb.mozillazine.org/Gmail