How to debug PowerShell Scripts which are executed via GPO during startup?

Problem was very stupid... I had added the Powershell Script in the Scripts Section not in the Powershell Scripts Section while in the Properties of Startup-Scripts...

I found out, that Start-Transcript does not create a logfile when executed as SYSTEM-Account (during Startup). You have to specify a path, then it creates the logfile. Sorry and thank you for your help.