Missing groups in post backup scripts


I have added my urbackup user to the tape (I want to write backups to tape in the post backup scripts), when I call groups in the shell everything is fine:

 # groups urbackup
 urbackup : urbackup tape

But in the post backup scripts the urbackup user only seems in it main group urbackup. For testing I call groups in the script and the output in the log is:

18.02.19 11:50  	INFO  	Script output Line(2): urbackup

Anybody knows why this is so and how I can change it?