May 21 2018 11:17 PMSolution
I solved the problem by moving the folder with the install files from the WindowsApps (which is hidden by default) into a user folder (e.g. Documents) and start the executable from there.
WindowsApps directory seems to have insufficient permissions for standard users after the OS Release in April . At least ubuntu is now running again without any problem. I am using 1803 Build 17134.1