The virtual path 'null' maps to another application, which is not allowed.

I have 2 URL's which I want to direct to a different server farm.
https://mydomain.com/* -> to my SBS2008 SP2 server (witch is in front and recieves all https requests)
https://mydomain.com/synergy  -> to EXACT a internal Server 2008 R2

I clicked on the server and added these URL Rewrite rules:

<rewrite>
<globalRules>
<clear />
<rule name="EXACT" enabled="true" patternSyntax="ECMAScript" stopProcessing="true">
<match url="synergy/(.*)" />
<conditions logicalGrouping="MatchAll" />
<action type="Rewrite" url="https://EXACT/synergy/{R:1}" />
</rule>
<rule name="SBS" enabled="true" patternSyntax="ECMAScript" stopProcessing="true">
<match url="mydomain.com/(.*)" />
<conditions logicalGrouping="MatchAll" />
<action type="Rewrite" url="https://XYZ-SERVER01/{R:1}" />
</rule>
</globalRules>
<rewriteMaps>
</rewriteMaps>
</rewrite>

....

<webFarms>
<webFarm name="XYZ-SERVER01" enabled="true">
<server address="192.168.0.1" enabled="true" />
</webFarm>
<webFarm name="EXACT" enabled="true">
<server address="EXACT" enabled="true" />
<applicationRequestRouting>
<loadBalancing algorithm="WeightedRoundRobin" />
</applicationRequestRouting>
</webFarm>
<applicationRequestRouting>
<hostAffinityProviderList>
<add name="Microsoft.Web.Arr.HostNameRoundRobin" />
<add name="Microsoft.Web.Arr.HostNameMemory" />
</hostAffinityProviderList>
</applicationRequestRouting>
</webFarms>

When I access http://mydomain.com/synergy I get the following error:

" The virtual path 'null' maps to another application, which is not allowed." 

Please help me... I'm stuck on this for days, now. I can't find any working solution online.

Thanks and kind regards,

Roy