I was in contact with Microsoft Support, having the exact same issue, and received this reply:
Thank you for reporting this error: This has been an issue since last week however it was already fixed last as well last week
Title: Affected users may see errors when using the Whiteboard
User Impact: Users may have received an error when using the Whiteboard feature in Microsoft Teams.
Final status: After a period of monitoring service health, we've determined that rolling back the affected update and restarting the impacted VMs has resolved the issue.
Scope of impact: Any user hosted in the impacted region may have experienced impact.
Start time: Wednesday, April 15, 2020, at 9:06 AM UTC
End time: Thursday, April 16, 2020, at 4:30 PM UTC
Root cause: A recent Whiteboard update intended to optimize the landing page for the Microsoft Teams experience inadvertently resulted in impact.
Next steps:
- We're reviewing our validation and update deployment procedures to avoid issues like this from occurring in the future.
This is the final update for the event.
Obviously this is not resolved as we're all also facing this issue.
Being unhappy with this answer I replied with this thread and the support engineer called me saying they will review the configuration for our tenant, and try to push the update to our tenant as well.
I'll update in 24 hours with the results of our tenant's config update.