No connection to Dovecot with external E-Mail Clients anymore

Virtualmin runs on my Contabo Webserver with Ubuntu 20.04 LTS. E-Mail with Dovecot and Postfix, all Packages are up to date. The Server runs now almost a year and until the End of March all worked well. Now i cannot access my Emails via external Clients like Thunderbird (78.7.1 64bit on Linux MInt 20) or Blue Mail what i use on my Android Phone. Roundcube Access works

I found out what was wrong

Can’t load DH parameters: error:1408518A:SSL routines:ssl3_ctx_ctrl:dh key too small: user=<>…

I fixed the issue 3 Weeks ago and i could receive E-Mails with clients again. But now since 3 days the Server again declines the connection to external E-Mail Clients with IMAP. I cannot find anything in the Server Logs about that. SMTP sending works but the Message is not saved in the Sent Folder and it complains that the Certificate does not match the Domain.

There are 4 Domains hosted on the Server.