Issue only "Default Page" after activating IPv6

[SiegbertG]

Hello,

I have Plesk Onyx 17.8.11 Update #71 on an ubuntu 16.04 server. I do the folling changes:

• go to server-administration tool from hoster
• activate IPv6
• wait until IPv6 is shown as activated
  
• go to Plesk -> Tools&Settings -> IP-Settings
• click on "read IP"
• I see the new IPv6-address
• configure the IPv6 as non dedicated (shared) address
• wait a while until all changes are valid

After that I call one of the urls hosted on the server and get only the plesk Default Page.
I tried "plesk repair web [domain]" and "/usr/local/psa/admin/sbin/httpdmng --reconfigure-domain [domain]" but with no success.

All sites use SSL with LetsEncrypt-certificates. The DefaultPage shows the ceretificate for the IP-address but not the certificat for the specific domain. So I tried to reconfigure SSL-Settings for this domain (disbale SSL, save, enable SSL, save) and to renew the certificat - but this leads to an error too (LetsEncrypt can't connect to the domain).

What do I have to do to make IPv6 work? Do you need some more information?

Thanks,
Siegbert