CloudSOC won't allow O365 Securlet activation using a new MS 365 account.
O365 Securlet was deactivated without purge.
Reactivate O365 Securlet with original MS O365 account using Global Admin following Tech Doc
After account has finished activation - click on the optional Purge box before deactivating again
CloudSOC should now allow using a different O365 Account to activate the O365 Securlet..
If reactivation / deactivation with purge using original account does not work contact CloudSOC Support.