I uploaded a basic sktech with a variable and it worked. I added a dummy variable to my sketch and ota update works now.
Now i have an additional question:
I have a second usecase where the opta is behind a firewall. I opened urls and ports following this description: Configure your network for Arduino IoT Cloud – Arduino Help Center
But i think for ota-update i have to configure addidional ulrs/ports?
Is there a description i can forward to my it-collegues?