Thanks for your replies!

First thing I did was to look for infos and I found that:
https://library.morph.zone/Applications/RDesktop

And I followed the notes and changed "Require use of specific security layer for remote (RDP) connections" to first "Negotiate" and then "RDP" (for a user wrote that this would work).
But, as I said, in contrast to the OS4 version - which runs but nevertheless both variants failed; I guess due to the gateway... - the MOS version seems to do nothing.

@roschmyr
Can you please explain those two options that more detailed for a novice...?! Where do I have to what to do...

@geit
Maybe you are right. But as said: The OS4 version works, the MOS not. So I have to look for the reasons and logically the initial starting point is to investigate error messages....