I am not able to create new Online mailboxes, because the on-prem mail users exist as contacts in the Exchange Admin Center.
When I try to delete the contacts there, the following error messages appear:
New Exchange Admin Center: "We have a problem with our server. Please try to resubmit your request later".
Old Exchange Admin Center: "Error checking in agent 'Windows LiveId Agent': 'Unable to save. 'Admin STEP' is outside of a valid server writing range."
When I tried to delete the on-prem mailuser with the command Remove-MailUser in PowerShell, appears again the same error message:
"Error checking in agent 'Windows LiveId Agent': 'Unable to save. 'Admin STEP' is outside of a valid server writing range."
Hybrid configuration isn't set up.