After I was still facing the same issue with the installation despite applying all hints, I completely purged the installation and re-installed/re-setup deconz. It uses the same versions as before (firmware 26780700, App in V2.19.3) but solved both issues:
- API Key Acquisition problem (Issue 2836)
- Internal error 951 issue
All state changes via GW UI and REST call are now working. Not sure what caused the issue with the earlier installation but I hope it is stable for now.
Thanks again for help!