BD is blocking sent mails to be saved in Thunderbird

Flisp

Whenever I send a mail, a copy is saved in the sent-folder. Now, however, I get the message that it took too long to connect to the server and that the mail is sent but no copy could be saved due to a network or fileadmission error (translated from swedish). Sending mail is no problem, neither i receiving. I use imap, Thinderbird 128.9.1 (64 bit) Win 10 and Bitdefender Total Security.

I have tried to turn off the Firewall, Antispam, Vulnerability, Online Threat Prevention and Ransomware Remediation to no avail. When I restart my computer mails are saved for a while, untill the problem starts again.
I ran out of ideas. Any suggestions?