Setting up new email queue - "Missing fields" error

Settings are as followed:

Queue type:Tickets
Mailbox type:IMAPSSL
Queue template group:Default
Email queue is enabled:Yes
Port number:993
Leave email on the server after fetching:No
Use the same mailserver to send email:Yes
SMTP connection type:TLS

This is identical to other queues. The connection verifies fine, but when "Insert" is clicked to create the queue, we get the error:

Missing fields
One of the required field is empty or...

Setting up new email queue - "Missing fields" error