I seem to receive the above dialogue box with increasing regularity. It
presents with my log-in and password details all pre-filled but clicking on
'OK' only succeeds in making it disappear for a few seconds before it
pops-up again. The only way of getting rid of it is to click on 'Cancel'
and it is then removed for 3-4 minutes before appearing again. In the
meantime I can't receive any e-mails (although I can send them OK).

I seem to experience episodes of this that often last several days and then
the problem goes away again for weeks/months.

Any ideas? Highly irritating!