"Mailbox isn't a valid MBOX file" Error

[Elise]

Running currently two local mailclients: Outlook 2021 and Thunderbird 
102.4.2

When using TB and remove a message on my main account, I get this error:
[CANNOT] Mailbox isn't a valid mailbox

On any other TB configured email accounts I can remove messages without 
any problems.
First I thought that Outlook might block removal on messages, but the 
issue appears as well if I close that mail client.

Can you tell what is causing this problem?

Best regards,
Jos Chrispijn