DPS installation removed ISAPI aspnet_filter.dll
There is a known Microsoft Bug that causes this issue:
Cookie-less session-state requests are broken when you install multiple versions of the .NET Framework on your computer.
Follow the instructions in this Microsoft KB article to reinstall this filter.
DPS 7.1 requires that the .NET Framework 1.0 SP2 or greater is installed on your machine. This is because some of our source code is written using the 1.0 FW. The DPS 7.1 installation will install the 1.0 .NET Framework on your computer during our installation if we determine that it is not already present.
If you are using the .NET Framework 1.1 and install DPS 7.2, it will not install the 1.0 Framework.