Hallo @Matthias_Hamburg,
This problem is based on the following recent change in the Let's Encrypt service:
The software that is failing to support TLSv1.2 here is not your web server (which accepts inbound connections), but rather your ACME client application (which makes outbound connections to the Let's Encrypt API service).
The relevant application is probably part of Plesk itself, or installed by Plesk, suggesting that Plesk would need to be upgraded to a newer version.