Degraded sync performance for Microsoft 365

Between 20:40-21:40 UTC we received responses from 365 from both EWS and Graph that told us the credentials we held were invalid.

Microsoft are reporting on this as incident MO1020913.

As this issue lasted longer than our quarantine period of 20 minutes, designed to ignore temporary and/or one-off issues, we considered the credentials we held for accounts attempting to sync during this period as truly invalid and so sent relink request emails to them.

Users that received such emails will need to go through the relink process as instructed by the email to fully reinstate their calendar synchronization.

We are continuing to monitor, our telemetry shows that sync operations to Microsoft 365 returned to normal levels by 21:40.

Because of this, some users may have erroneously had their credentials invalidated and so they will need to relink their accounts.

Microsoft are reporting on this as incident "MO1020913".

We are still investigating, but we have also seen sync operations begin to return to normal levels of failures. The majority of syncs are now successful

We are currently investigating sync failures to Microsoft 365. The majority of syncs are resulting in failure, and both Exchange Web Services and the Graph API are affected.