Ensure the following steps are followed:

1. Check the firewall policy to ensure access to Microsoft is allowed.
2. Try pinging the Microsoft site to ensure it is reachable.
3. After verifying that the Microsoft site is accessible, re-authenticate the M365 account.

To re-authenticate Run on Client backup sets:

1. Open AhsayOBM and click Backup Sets then select the affected backup set.
2. Go to General then click Change settings.
3. Click Next then Authorize to start the authentication process. 

To re-authenticate Run on Server backup sets:

1. Open AhsayCBS and go to Backup / Restore > Users, Groups & Policies.
2. Select the affected backup user then go to Backup Set then click the affected backup set.
3. In General, click Change settings.
4. Click Next then Authorize to start the authentication process.