I think I figured it out: Server Name Idication was not turned on, because this Plesk was updated from some older versions.
To, fix, change set "SNI_SUPPORT" to "true" in the file /etc/psa/psa.conf
I just updated to Plesk 12.5 on Debian 7 Wheezy.
No matter what I do, I can't get letsencrypt to work, or, better said, I can't get Plesk to actuall use the certificates that the letsencrypt extension generates.
I can see the generated files in...