Hey Thomas, thanks for your reply.
Today I checked everything twice and even did a total clean install of PostgreSQL, Jetty and XWiki.
However the problem still persisted. Logs were clean and I had no clue what causes the problem.
I also increased CPU and RAM since it runs on a Virtual Machine and tried a total non-customized install in the standard jetty.base
Then after fiddling and tweaking for a while I found a rather unusual solution 
- Start XWiki Setup and wait until first dialog is loaded
- Close browser
- Open browser and call XWiki URL again
-> Admin Account creation dialog shows and no more permission problems during flavor installation
Here you have some more screenshots which show some weird thing going on with the variables:
Maybe it helps…


This is the exact system setup I’m using:
WIN Server 2012 R2
Jetty 9.4.9.v20180320
PostgreSQL 9.6.8
Java 8u162
Firefox 59.0.2 (64-bit)
JDBC Driver postgresql-42.2.2.jar
JIRA gave me
https://jira.xwiki.org/browse/XE-1371
Do you think it could be related?