Remote Desktop app can't connect through VPN or through RDP load balancer

Using the regular Remote Desktop Client (in the desktop environment) I can connect just fine to remote servers when connected through Cisco VPN or when accessing a server behind a load balancer.

When using the Remote Desktop app in the Modern UI, I can't do either of these things. Trying to connect to a remote server that's on a private network fails with:

Can't find server, make sure the name and domain are correct and try again

And connecting to a server that's behind an RDP load balancer fails with the following error, after accepting credentials:

Because of a protocol error, this session will be disconnected. Please try connecting to the remote PC again

Is there some way to use the Remote Desktop app in these situations, or am I just out of luck?

I had the same problem. When I connected via VPN it wouldn't connect however via the desktop version it did. I access a lot of servers and my office PC from home when I updated to the RDP UI it stopped working. Just had to add my VPN default gateway to the settings in RDP and it worked every time. Hope this works for you.