Thanks for your suggestion.
Plesk writes about this problem in a help topic too ==> SSL 301 redirection for domain alias
But, unfortunately, the information given by Plesk is wrong in two points:
1.) The Alias http://alias.com for the main domain https://example.com is not redirected to...
How to make a 301 redirection from the Alias Domains to the Main Domain, which has "https://" and "www." too?
There is an option in Plesk for the Alias Domains, that is called "Redirect with the HTTP 301 code". But this option doesn't work, if the Main Domain is SSL protected (https://).
In...