ArcIMS 9.1 Service Pack 2 is currently not certified for use with Route Server 9.1.
With ArcIMS 9.1 SP2 installed, Route Server 9.1 fails to start and the following error message is returned in the Application Server log:
<ERROR code="1" numthreads="0" >
library can not be loaded : C:\Program Files\ArcGIS\ArcIMS\Server\ext\RouteServer\RouteServer.dll
Updates made to aimscsys.dll in ArcIMS 9.1 SP2 causes the Route Server to fail during initialization.