OK - you wrote: There's a lot of reading to do
but I think that was in the meaning of: There's a lot of understanding to do

Be sure, I'm not too lazy to read and I understand what I read very well (although my written English is really not good).
Edit:
raising a ticket is not possible because I'm a custemer of the german company Strato and they deny support for Plesk although they are a trusted partner of plesk.
Yep, there is a lot of reading / understanding if you are going to go through all of that lot on your own and... it is all in English unfortunatley (in your case), but your written and commmand of English is just fine and 1,000,000% bettter than our knowledge of German

as a comparison.
The support ticket IS very easily possible. It doesn't matter that Strato is a Plesk Partner. You may have misread that page? Here's the relevant lines from it:
"...However, if you would like to get support directly from Plesk, you may purchase the Plesk support subscription..."
Followed by:
"...How can I purchase it?"
And then what people sometimes don't see / don't read properly:
"...Subscription has a free trial period for 1 month. If you order Plesk support subscription for a license for the first time, then you will be charged $0 for the first month and $10 for each next month..."
IONOS is our Cloud Server provider & they are a Plesk Partner. We took the above subscription just to cut out the middleman / delays etc. It's all 100% ok. Finally, Strato can't "deny" you
independent verbal and/or written support on anything from anybody, they are not the
Staatssicherheitsdienst - SSD - Stasi

They
might... forbid indpependent, direct root access to your server
as part of your contract, you would have to check, but how would they police & manage that (and why would they want to) anyway, if you alreay have that access level yourself? Plesk can assist via direct root access, as part of their ticket support process via their ssh extention:
Support SSH Access if/when it is needed, which it may not be. You're both still not sure (at this stage) what's required to solve this.