[Techtalk] Evolution won't talk to IMAP server

[David Sumbler]

I use fastmail.fm for email, mainly because I can sort mail into folders 
on their server.

I access my mail by IMAP using 4 different machines:

1) A desktop computer running Ubuntu 12.04 and Evolution 3.2.3 and 
connecting to my router by Ethernet.

2) A netbook with exactly the same versions of Ubuntu and Evolution, but 
connecting to the router by wifi.

3) A Google Nexus7, connecting through the same router via wifi.

4) A Samsung Galaxy S2, also using wifi, but running a different Android 
version and a different mail client from the ones on the Nexus.

Since yesterday morning, whenever either of the Ubuntu machines tries to 
update emails in Evolution, it appears to run normally.  But when I try 
to view any of the folders on the remote mail server, I get a "You must 
be working online to complete this operation" error.  I am, however, 
online even according to Evolution's own dropdown "File" menu.

Both my Android machines continue to be able to access mail normally.

I also find that Thunderbird, on my main (desktop) machine works 
absolutely fine.  Firefox, too, works as normal, so the problem appears 
to be only with Evolution.  I have not altered any settings recently.

Also Evolution itself has not been updated lately - my version is 
already the latest from the Ubuntu 12.04 repositories.

So how is it that, apparently simultaneously, 2 machines running 
Evolution suddenly and independently stop being able to refresh any mail 
folders?

It doesn't seem to make any sense (to me!)

Can anyone think of what I should try next to sort this problem out?

Thanks for any suggestions.

David