I recently upgraded a customer from 8 to 8.5 to fix an issue and have run into a new one.
For three agents I've gotten the error message: "Login failed due to a configuration error with your phone and JTAPI or Unified CM. Contact your administrator."
For two of them, the fix was to un-associate and re-associate their device with the RmCm user. For the last fella, I've tried complete rips and rebuilds and I can't get him past the error message.