I managed to find the solution:
I simply threw the whole error-message into ChatGPT5 and asked for a solution. Through suggestions, I narrowed it down to that the package libharfbuzz0b was missing in the docker image.
I did a “apt-get update && apt-get install -y libharfbuzz0b” in the exec console for the container in Portainer. It was indeed missing - and installed without problems. After, I restarted the container - and now have charts.
(rain since midnight - and yes, a good day to tinker with Openhab today)
Since I already have a custom image build, based on openhab/openhab:5.0.1, I have simply added it to be installed when building the image. That has also worked - the chart above is from after having rebuilt and updated the container.
Perhaps the above can help someone else ?