I've had this arrangement, stable, for a long time - years.
I have 2 email addresses.
Mailwasher Pro recognises and downloads all emails to both addresses.
Thunderbird downloads one set of emails, but then for the second address I get the message, bottom left, "Downloading 1 of 28" but nothing downloads. Clearly it "sees" the emails but something is blocking the download.
The following message implies a B9 problem, as that's the toolbar label associated with it:
"An internal error has occured while communicating with the mail client (EIdNot Connected: Not Connected) Please contact Technical Support with a full description of the problem"
Do you recognise this as a B9 fault message? If so - where next?
I don't think it's a Thunderbird problem because surely that would affect emails to both addresses. But just to check, I deleted the problem account and reconstructed it - same problem.

regards
John Wilson