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No error message available, result code: DB_E_ERRORSOCCURRED(0x80040E21)

One of the least useful errors I've come across. I came across this whilst making some changes to an old website which was using OLEDB. I restored the DB locally and set it up to user Integrated Security rather than reinstate the permissions from the server.

Apparently this was the problem - rather than using Integrated Security=True in the connection string, to get this working with OLEDB, we need to use Integrated Security=SSPI. A quick change to the web.config file and we're away!