After hours of googling I finally found users with the same problem.
I create daily automated backups to Strato HiDrive, one via Plesk and several via a cronjob on my server, which uploads with a curl command.
Since several days or weeks I always got this error from Plesk:
pmm-ras failed (Error code = 1): Repository error: Failed to read backup backup_2003160309.tar: Import error: Unable to find archive metadata. The archive is not valid Plesk backup or has been created in an unsupported Plesk version
Today I found out that not a single backup was uploaded correctly. Neither via Plesk, nor via my script for curl. Even manual uploads via console are incomplete (and have a different size with every upload).
I just did a verbose output of curl and it said (multiple times):
TLSv1.3 (OUT), TLS Unknown, Unknown (23)
If I understood correctly, both Strato HiDrive and Hetzner Storage Box (which I wanted to try now) don't support modern encryption, which curl uses by default.
EDIT: Today I made several tests. Concerning STRATO HiDrive I found out that using curl with --tlsv1.2 is neccessary. Only then the files are complete. If I use only --tls-max 1.2 (which should be enough), it does not work, because TLS 1.3 is still used. To make it even more safe, I use now --tlsv1.2 --tls-max 1.2.
Concerning Hetzner Storage Box it seems that TLS 1.3 ist not supported at all, only 1.2 (and maybe lower). Did they recently deactivate 1.3, because it was broken? When I do not specify the TLS version, 1.2 is used. If I force 1.3, I get an error and no upload is done at all. I did not get an error message from the Plesk backup of last night, but the remote backup has another file size than the local backup. Or is this correct? The remote file is larger.