Mail stuck "moving messages" to account folders with Mail rules

Hi,

I'm on macOS Monterey 12.2.1 and have a problem with the built-in Mail app. I use a lot of mail accounts. But I move most mails to folders in my iCloud and Office 365 accounts with Mail rules.

The problem: Mail is stuck on an ever-increasing number of mail messages that it can't move. They show up in the destination folder but have no content. Here's an example of a German email that shows up on my Office 365 inbox on iPhone:

Now Mac Mail failed to move that message. So it shows up with an empty body:

At the same time, Mail's "Activity" window shows an ever-increasing number of messages haven't moved:

I quit Mail and used "CleanMyMac X" to "Speed Up Mail" (which I think rebuilds some index here). I selected "Mailbox - Rebuild" in Mac Mail with both my iCloud & Office 365 inboxes selected. The latter is still ongoing, but none of it has made any difference so far.

I had this problem in the past, too. But it always sorted itself out over time and possibly some Mail restarts.

How can I get Mail unstuck?

Regards,

Karsten S.

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