I'm confirming the issue about copying from the server to the client. Actually, if you want to copy something from the server to the client, you just need non-empty clipboard in the ubuntu session (you don't need to copy something to the server).
How to reproduce:
1. Logout from rdp and ubuntu sessions.
2. Login to ubuntu.
3. Login to rdp session.
4. Try to copy something from rdp session to ubuntu.
If you fill ubuntu's clipboard with something after step 2, you will be able to copy from the server.
I'm confirming the issue about copying from the server to the client. Actually, if you want to copy something from the server to the client, you just need non-empty clipboard in the ubuntu session (you don't need to copy something to the server).
How to reproduce:
1. Logout from rdp and ubuntu sessions.
2. Login to ubuntu.
3. Login to rdp session.
4. Try to copy something from rdp session to ubuntu.
If you fill ubuntu's clipboard with something after step 2, you will be able to copy from the server.