Sent message remains in Drafts and is not saved in Sent folder

I have a recurring problem with Thunderbird 153.2.0esr (64-bit). After clicking the Send button, the message is successfully sent to the recipients. I can see that the sending process takes place, and I have confirmed that the message was actually delivered because I was one of the recipients myself and several BCC recipients have replied to it. However, after sending, the message sometimes does not appear in the Sent folder. Instead, it remains in the Drafts folder. This is particularly problematic because it gives the impression that the message may not have been sent, although it actually has been delivered. There is therefore a risk of sending the same message again by mistake. The problem occurs only occasionally. The most recent occurrence involved an email sent to approximately 180 recipients via BCC with an attachment of about 1.8 MB. My email provider is WEB.DE and the account is configured as an IMAP account. In Thunderbird under Account Settings → Copies & Folders, the option to save a copy of sent messages in the Sent folder is enabled. Expected behavior: After successful sending, the message should be removed from Drafts and a copy should be stored in the Sent folder. Actual behavior: The message is successfully sent, but remains in Drafts and no copy appears in the Sent folder.