Updated Question:
Note: Even the updated question is still misleading, please read the answer for the real issue.
I have a strange problem. I'm connected to my network via a VPN-SSL client (Fortigate). I can not access any machine via RDP directly through its IP address. It works only via a hostname (or domain name since it's a local domain environment). I can ping the systems via the VPN connection, as well as use other services (e.g. MSB) through the IP address. But clicking "connect" on the RDP-client (mstsc.exe) simply does nothing, not even an error message.
The VPN-network connection is routed, but as I said, other services work fine so I don't suspect a routing/firewall problem here. And I don't have any rules which apply specific to RDP connections (I'm talking about the Fortigate firewall appliance here, not the Windows firewall from the client. The Windows firewall on the LAN-client I'm trying to connect to is disabled).
Could there be some RDP server or client settings I missed? Any help would be appreciated.
Note: Using RDP via IP on a different machine/VPN-account works, so it could also be a problem with my local machine or VPN account. I'm using a faily up to date version of mstsc.exe (10.0.18362.1316).
Original Question:
I have a strange problem. I'm connected to my network via a VPN-SSL client (Fortigate). I set up a Win 10 test system which I want to access via RDP. I can ping the test system via the VPN connection, but clicking connect on the RDP-client (mstsc.exe) simply does nothing, not even an error message. Connecting via LAN (and not VPN) works fine. -> PLAESE NOTE UPDATE1: it does not work with any remote machine via IP
This seems to be RDP-protocol-specific (possible in connection with the VPN). I set up a second Linux test system with xRDP. Connecting via SSH through the VPN works fine. Connecting via RDP through the VPN results in the same problem. Connecting from the LAN works fine.
The VPN-network connection is routed, but as I said, the ping works fine so I don't suspect a routing/firewall problem here (I'm talking about the Fortigate firewall appliance here, not the Windows firewall from the test client. The windows firewall on the test client is disabled). Could there be some RDP server or client settings I missed? Any help would be appreciated.
Note: This is just via IP no DNS involved
Update1: I just realized the RDP behavior is persistent when I try to connect to any remote machine via IP from the VPN. But I can use the IP to establish file access etc. Also RDP works fine as long as I use the hostname/domain name (the LAN is a domain environment, so I use FQDNs: hostname.domainname.local). Strangely this problem does not appear on all the clients, using a different PC/VPN-account the problem goes away. I suppose I have to pinpoint it a little further. I'll try to access via IP the next time I'm physically connected via LAN.