Solution

The cause of the problem was the SysOperationProgress class. This class can be used to inform the user about status, operation and progress of the system. It was initiated before my changecompany call and closed after it. When the progress bar created by the SysOperationProgress class was refreshed during code execution within my changecompany class, a call is done by X++ to the Docu class. This triggers also a changecompany due my original legal entity. Hence some of my code within my changecompany call was executed in the company to which I did the change, some was done in the company I originated from. And this not all of the time, as it only happened if a refresh happened of my progress bar within the changecompany call. I removed the SysOperationProgress and my problem was solved.