Hello @Markus Geiger ,
Thanks for posting your query on Microsoft QnA.
I would like to share a word of caution before answering the question for your specific scenario. It is important that you don't repeatedly start/stop your cluster. Repeatedly starting/stopping your cluster may result in errors. Once your cluster is stopped, you should wait 15-30 minutes before starting it up again.
In your case, I understand that the cluster is stuck is "starting" state for hours when you start the cluster from Portal.
I would recommend you to perform the stop as well as start of your cluster via cli using the below commands. If you still see the same issue persisting, then kindly drop a comment here and tag me so that i can look further into this. Please note, starting and stopping a cluster via cli also takes a few minutes. If the nodepool size is small then it should not take more than 5 minutes (this is what I have observed from experience).
az aks stop --name <cluster-name> --resource-group <rg-name>
az aks start --name <cluster-name> --resource-group <rg-name>
Hope this helps. Looking forward to hear back from you. Please do tag me in your reply. Thank You.