this error was seen before by other users. In the middle of the migration process I was suddenly no longer be able to access the old page in the V2 console. The device stopped to be accessible an no data passes from that moment on.
The full Error Message was:
Error: permission denied: Could not re-register missing device to Broker: permission denied: NetworkServer did not set device: permission denied: V2 clusters are now read-only. Migrate your devices to a The Things Stack (V3) cluster now! V2 clusters will permanently shut down in December 2021.
My way around was to demount the sensor and reconfigure it with another AppKey. I report this here by request of someone in the Support Team of TTI and hope that they can do something for others that are not enabled to access their OTA-Nodes as I was when faced with the error.