If you run into those issue, first run
gpresult /h c:\gpreport.html
on your client, then open the file with your browser and search for the application you want to have started on user logon.
In my case, I found out this way that one group policy object had simply overwritten the values of another group policy.
If you have multiple GPOs and set this property multiple times, the applications to start are NOT appended and all started. Only the most important GPO „wins“.
So I suggest – for multiple policy objects – not to use the „run programs at user logon“ feature at all.
Instead, use startup scripts. They can be created in multiple GPOs and they are all applied.
Simply create a .bat file with content
and add it to your policy.
Another way could be adding a link to your start menu startup folder, but you could run into language issues (Autostart, Startup, …).