IMAP settings will not persist

IMAP settings will not persist

I am trying to enable IMAP access on my email account. I have gone to Settings -> Mail -> Email Forwarding and POP/IMAP, and under IMAP Access I have clicked Enable. However the setting will not persist. As far as I can tell the setting is not being saved. When I leave the page and return, it is marked Disable again.

How do I save the Enable setting once and for all?

Thanks.