RDP Settings Domain & Username / Wyse 5070 / IoT 1607 / BIOS 1.5.0

When I create an RDP Server Icon, no Domain Field appears at startup. (just a grey area)
Default Domain is configured in WMS (v1.4.1)
All Users have to use "user@domain". Also last Username is not stored and the RDP Security Warning appears too.
What I am doing wrong ?

Can I upgrade to LTSC 2019 ?

Any help is highly appreciated....