Connect-Pnponline not working

[ToasterSch]

Hi,

i have a SharePoint 2019 Farm and on my 2 Frontend Servers Connect-PnPOnline is not working properly.

Connect-PnPOnline -Url https://it.our.realm -CurrentCredentials throw Object reference not set to an instance of an object.

Connect-PnPOnline -Url https://it.our.realm -Credentials (Get-Credential)
with Credentials our.realm\user and password brings User realm discovery failed
with Credentials user@our.realm brings The user account {EmailHidden} does not exist in the psw-engineering.de directory

Do i miss an dependency? Or do you have a hint for me?

The whole Installation is on premise, no Cloud involved

BR
Thorsten