Issues Setting Up Standalone EAS Proxy with Exchange Online in Powershellmode

Hi Guys,

i'm in a situation, ehre i have to set up a standalone eas proxy in combination with Exchange Online.
the struggel begins, when i try to set up the instance.
i prepared the service user as mentioned in the sophos knowledgebase.
however, when i try to click "add" in the Configuration Wizard i receive the errormessage "Cannot connect or login to outlook.office365.com. Please check your input".
I'm able to login trough a browser from the Server.

The Version of the EAS proxy is: 9.7.5.54
The Powershell Version on the Server is:5.1.17763.2803

I already made sure, that TLS 1.3 is activated on the Server.
Further i checked on the Firewall an disabled TLS Inspection an Webfiltering.
The Firewall Log indicates, that all traffic regarding this problem was allowed.

I am very thankfull for any tips!

Edit tags
[edited by: GlennSen at 6:00 AM (GMT -7) on 24 May 2022]