Just to give some feedback after we solved it by our selfes:
We removed the old installation and imported the cluster again in the new installation, worked fine wihtout problems.
We also had another migration, where we moved again since our old rancher installation didnt work anymore (on a managed kubernetes cluster, which is not recommended) and wasnt also reachable anymore.
we just imported the cluster into the new rancher installation, without removing it from the old one. Everything worked fine also, except that the projects-namespace associations were broken and we had to reassign all namespaces to new projects again. Also somehow our alerting is still not working correctly, the notification emails still point to our old rancher installation domain, even though we changed all config files which we found.
