This whole thing started very simply. Install a Office Web Apps Farm for a Lync 2013 deployment.
We are using Server 2008R2 SP1, fully patched. The Web Apps Server (WAS) installed correctly, or so it seemed. But then the farm failed to create. This was tracked down to MS KB 2670838 getting in the way of installing MS KB 2592525 – removing MS KB 2670838 solved that – with a weird side-note – removing 2670838 also removed IE10. There is ample evidence on the web that 2670838 is dirty dirty dirty, bad bad bad. Things are starting to go sideways here!
So we finally got the patches straight, MS KB2592525 goes in. Now the farm created, but then we could not hit https://fqdn.domain.com/hosting/discovery with an error of 500.21, and a claim of “ http error 500.21 handler “discoveryservice” has a bad module managedpipelinehandler” … El Yucko!
After some searching back and forth, it seemed that asp.net was either not installed or was not registered properly. Looking at the installed set of programs and features on the IIS indicated that ASP.net was indeed installed….
What was needed was to reregister the ASP.