Hello Arigato, I am having a similar issue with error 0x204.
I have tested iPad connected to wifi via my Verizon hotspot, iPhone as a hotspot, and two other backup cellular connections. If I connect to Verizon, or either of the two backups (all 3 of these have IP addresses that start with 192.168.1.....) and then connect to my company's VPN the RDClient app gives me the Error code: 0x204. However, when I connect my iPad to my iPhone hotspot (IP 172.20.1......) and the VPN, RD Client connects. I mention the IPs because the desktops I'm attempting to connect to are also on a subnet that coincidently start with 192.168.1..... My Network Admin thought that may be playing a part of the issue.
I tested these same 4 wifi connections in conjunction with VPN on my windows laptop using the native RDP and all combinations worked.
Alyc