Thanks for all the suggestions.

For the affected terminal server I made a resulting set of policies; no policies are set for logon events / behaviors.
The scripts fire at the same millisecond; when a policy and a user logon script where configured i'd expect some difference in start time.

Exiting the usrlogon script at line 1 didn’t make a difference.

Since the logon script firing twice the “My documents” folder pops up in explorer. No reference found to that action either in GPO, run keys etc. ‘thought I mention it.