Modern Auth EWS error 50199 when signing from Crestron Touchpanels

Good Afternoon, All

I am having a difficult time nailing down this issue. 

I have a few Crestron TTS-770s that were, up to last week, working correctly by pulling Calendars data with EWS. They were configured with a service account signed into EWS using 'modern authentication'. 

This week, these panels have disconnected and report that 'Needs to be authorized' as the EWS status. I have verified that CA is not blocking sign in, the account is excluded from MFA policies, and is correctly licensed for Exchange Access. We do not use Intune for device management.

When I attempt to re-register the device, I follow the prompts until I am prompted to close the browser window; The device spins, then fails to connect with the status above. I have attempted this with the service account and my own Admin account with MFA, to the same result.

Entra Enterprise Apps Sign-in logs show a 'Successful' entry, then immediately after, a 'Failed' entry with an error '50199'.

I had not made changes to any of the URIs before initial failure, and any additional entries or changes do not change the results of the error. Initial URI was configured to 'https://app.noop' (no idea, was configured before I got here, and I hadn't needed to change it), I have attempted combinations of our Tenant URI, '

https://login.microsoftonline.com/common/oauth2/nativeclient',    and other 'fixes' I had found while GTS-ing.

I additionally have set my 'legacy authentication' and 'legacy applications' CA polices to read-only for troubleshooting.

I am working to disable OAuth2ClientProfile on Exchange Online temporarily for troubleshooting.

Does anyone have any ideas? Please let me know if any additional information is needed, or if needed to post in another location.

Thank You