[Evolution] No data source found for UID “8ea059fe1c3d74abd4698ccdf3bbd12269378c04”

[Keith Barber <keith keith barber name>]

Using v3.36.4-0ubuntu1 on ubuntu 20.04.

New installation following other email clients blowing up on me.
 Following problems one of my email hosting companies suggested using
IMAP to collect mail from them... all new to me but it seemed to solve
the immediate problem.  However, I now get the above error every time
Evolution tries to collect mail.

Support people at the host say this message comes from Evolution rather
than from the server.  (They do acknowledge the possibility of a
server-side error triggering it, but at this stage I don't think it's
particularly likely; they'd have sorted it by now, I suspect.)

They do ask if Evolution has an option to rebuild a mailbox index.  I
couldn't find any such.

So two questions:
        1       Is there an option lurking somewhere to rebuild an
index?
        2       Any ideas what is going on with this message and how to
rectify the problem?

Thanks in advance,
Keith Barber
(he/him)