HTML5 VPN Portal - RDP error

Hi Guys, 

I have a recurring problem connecting to MS 2008 R2 Terminal Servers via RDP using the HTML5 VPN Portal. Most of the time the client can connect to the server and sees the login screen appears (so no network issues present, RDP is working initially, no misconfiguration with NLA or TLS) and can provide credentials. However AFTER pressing Enter, the screen freezes and after 10-20 seconds the session terminates.

Below are the corresponding logs:

2017:01:25-08:15:55 fwhostname screenmgr[11250]: Client 4: authenticated: user='user@domain.com'
2017:01:25-08:15:55 fwhostname screenmgr[11250]: Client 4: start screen requested: REF_CliCon3pmdhpaxap10
2017:01:25-08:15:57 fwhostname screenmgr[11250]: Client 4: connector started
2017:01:25-08:15:57 fwhostname screenmgr[11250]: id="2201" username="user@domain.com" event="Connection started" sub="vpn" variant="clv" sys="SecureNet" severity="info" srcip="ip.add.re.ss" service="HTML5 VPN" type="VNCRDesktop" sessionid="4" sessionname="SESSION-NAME"

2017:01:25-08:16:10 nkvnfwlp05 screenmgr[11250]: Client 4: connector stopped: 1
2017:01:25-08:16:12 nkvnfwlp05 screenmgr[11250]: Client 4: 400: Bad Request: Bad method: vnc
2017:01:25-08:16:12 nkvnfwlp05 screenmgr[11250]: Client 4: 400: Bad Request: Bad method: vnc
2017:01:25-08:16:12 nkvnfwlp05 screenmgr[11250]: Client 4: 400: Bad Request: Bad method: vnc
2017:01:25-08:16:12 nkvnfwlp05 screenmgr[11250]: Client 4: 400: Bad Request: Bad method: vnc
2017:01:25-08:16:12 nkvnfwlp05 screenmgr[11250]: Client 4: disconnected: Broken pipe

Also debug logs show no additional info regarding what the problem could be.

Normal connections with the RDP clients to servers have no issues. UTM9 firmware version is 9.407-3.

Anybody got any idea?

Thanks

Attila