Citrix through DSL/VPN not working

We have a VPN setup using a WatchGuard firewall and a Citrix Metaframe Server.  Remote clients connect by initiating their Internet connection, Connecting to the firewall, and then launching the ICA Client to connect to the Citrix Server and receive a remote desktop.  This setup works in every configuration tried so far except the following.  If the client is Windows XP using a PPOE DSL Modem connection the client can connect to the VPN but when accessing the Citrix server the client connects but does not display the login window for the user to login and receive their remote desktop.  Traffic can be seen passing back and forth but eventually the connection to the server times out as no login is ever provided.

Any suggestions?
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DSL with PPPoe uses just enough extra overhead to cause fragmentation issues.
Solution is to reduce the MTU of those PC's behind DSL..
Suggest using 1300 vs default 1500

Google for DrTCP utility to set the MTU.
If they have a small router, the router may have an option to change it on the router. Change that to 1492

You can also try setting it on the server itself. Set server's MTU to 576

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