While logging in to office applications CAA90049 comes up. This used to be only fixable by changing a REG to turn modern Authentication off. (HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity:“EnableADAL”=dword:00000000)

However this is not advisable since this will either be decrepiated or lower the security of the particular systems logins with M365. The cause of this can be that the AAD Broker Plugin might be missing. To fix this take the following steps:

1. Run “Get-AppxPackage Microsoft.AAD.BrokerPlugin” the folowing command in Powershell
2. This should return if the AAD broker plugin exists, if this is missing it will return nothing
3. Run “if (-not (Get-AppxPackage Microsoft.AAD.BrokerPlugin)) { Add-AppxPackage -Register “$env:windir\SystemApps\Microsoft.AAD.BrokerPlugin_cw5n1h2txyewy\Appxmanifest.xml” -DisableDevelopmentMode -ForceApplicationShutdown } Get-AppxPackage Microsoft.AAD.BrokerPlugin” In Powershell
4. If needed re-enable ADAL in registery and reboot. This should install the missing plugin resolving the CAA90049 error.

Per referance while available: https://community.spiceworks.com/t/office-365-wont-log-in-without-enableadal-0/707477/5