So long conversation with Mr. Google.
The garbage that comes up on these searches these days is ridiculous It takes you hours to sift through all the dross.
- Change the IIS pipeline mode to Integrated instead of Classic for the site.
- Send a dummy Name claim.
- How To Resolve HttpException (0x80004005): Failed to Execute URL
The last one looked like a no-brainer because it specifically talks about a site behind ADFS. Except ... it didn't work.
The second was the suggestion from:
Failed to Execute URL Exception when on windows 2003 relying party with IIS 6 .
@paullem suggested that it was because the Name claim was null. The fix was to add a dummy claim inside CAM (the Claims Authentication Manager).
I was using ADFS and the Name claim wasn't used to I simply ran up a static claim i.e.
=> issue(Type = "http://schemas.xmlsoap.org/ws/2005/05/identity/claims/name", Value = "Joe Bloggs");
That seemed to do the trick.