I did not finaly figure out what it was but was able to revert back to a backup-state that worked.
Ended up just creating a new Grafana App and manually transfered all settings. Not it appears to be working fine.
So probably I screwed up somewhere but it does not seem to be a bigger Application issue.
just had the same issue and wanted to add something here as a documentation:
If both Grafana and Prometheus are running as a cloudron-app, you can add Prometheus as a data-source in Grafana by inputting http://<PROMETHEUS-SERVICE-UUID>:9090 in the URL-field where PROMETHEUS-SERVICE-UUID can be found form the prometheus container either by looking up its installation location in the Storage-Tab or by connecting via terminal and copying the hostname.
@ChristopherMag This should be fixed now with the latest package. I found that we have to set GF_PLUGIN_DIR env var or pass --pluginDir separately for the CLI tool. The server reads configs from custom.ini but the CLI does not (there's probably some good reason for this discrepancy).
Right, /app/data/custom.ini is where the users customizations go. What db were you using in your existing installation? The cloudron package uses postgres. To get the DB connection credentials, you can go to the Terminal of the app and do env | grep CLOUDRON_POSTGRES. You can also click the postgres button on the top and it will paste the CLI needed to access postgres.