After Test multiple Version and told by Dell that we wont get any support for the Software with only Basic Service I found the issue.
After upgrading WMS to 3.8 there is a new option for a 30 Minute Timeout, which is enabled by default.
I was not able to see this option before the WMS Update but it seems the client had this turned on somehow.
However if anybody stumble about this the option is here (our system is German so maybe the translation is not 100% correct):
Your Settings Group->Edit->ThinOS9->Advanced View->System settings->Settings for Operational and sleep mode->Power connected->Timer when plugged in (default 30 Minutes)