An IMAP mail system is configured on a server running Debian 12. While there are never any problems via webmail, reception and sending work occasionally via Thunderbird 115.9. Even in "Troubleshooting Mode" it doesn't work. The server has good hardware characteristics and there are no applications that can slow it down, nor do I think it is a firewall problem or incorrect parameters otherwise it would never work. The error is related to timeout reaching the server. What can I check to find out where the problem is?