[Users] Get Mail button stopped working

[Adrian]

The Get Mail button at top left has stopped working.  I have the
Network log window open, and when I click Get Mail I see no network
activity. All that happens is that the bottom bar briefly displays
"Scanning folder Mailbox/<each folder>" (Mailbox is an 'account' of
local MH folders).

If I click the dropdown to the right of Get Mail and click on any of my
IMAP accounts, I see FETCH etc lines in the network log and I receive
any new mails.

In the past Get Mail would do FETCHes for my IMAP accounts, but this
has stopped in the last few days.  Thanks for any advice.

Claws Mail 3.16.0 under Ubuntu 18.04.