Microsoft 365 Apps admin center devices are not checking in.

[ChangeOnlyFridays]

None of our devices have checked in since August 2nd. I see no obvious blocks in our FWs or AV systems.

[#2]

Two quick thinks to check on an affected device:

Is the TAK (token to connect) set: HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\cloud\office\16.0\Common\officesvcmanager > TenantAssociationKey

Check the value of HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\C2RSvcMgr > LastCheckInResult. Is it 200 (successful check-in) or a different value?

[#3]

I have checked a couple devices. None of them have a TenantAssociationKey. Some devices have LastCheckInResult as 500 and one has 401.

[#4]

Are you positive that the devices are meeting the requirements as listed here: https://learn.microsoft.com/en-us/deployoffice/admincenter/inventory#requirements-for-using-inventor... as well as that users are signing in with an AzureAD account from your tenant and have launched the Microsoft 365 Apps at least once?

[#5]

@ChangeOnlyFridays 

How about try another type of browser and InPrivtae mode?