I did a bit more work on this yesterday and thought that the problem could be related to byte swapping between the Crypto API’s and the .NET managed code wrappers as discussed here: (http://blogs.msdn.com/shawnfa/archive/2005/12/05/500144.aspx).
It wasn’t! So today I gave up and did an IPCS re-install expecting problems. Guess what – I ran IPCS setup.exe, the license key was re-generated and the existing EMPS configuration data was maintained.
I should have done this in the first place. There again, I now know a bit about cryptology which I’m sure will be of use sometime in the future.