HTML 5 VPN Portal issue

Hi,
I've a problem with HTML5 VPN Portal. 4 of 6 RDP Connections are working. 
2 of them are showing "internal server error". RDP is working to this server, firewall disabled. Operating System is Windows server 2003 R2.

2014:05:06-08:30:36 SPMUTM01-1 screenmgr[4712]: Client 28: authenticated: user='Remote'
2014:05:06-08:30:37 SPMUTM01-1 screenmgr[4712]: Client 28: start screen requested: REF_CliConBsaxmdb12
2014:05:06-08:30:37 SPMUTM01-1 screenmgr[4712]: Session[28]: Service has stopped with an error: 
2014:05:06-08:30:37 SPMUTM01-1 screenmgr[4712]: Client 28: session stopped: 
2014:05:06-08:30:46 SPMUTM01-1 screenmgr[4712]: Client 28: disconnected: Broken pipe

Anyone an idea?

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