Yes, I found that same explanation, but it doesn't seem to fit here. If it's non-clustered index corruption, how do you explain:

1) There is no non-clustered index on any table affected direclty or indirectly by the SP that causes this error.

2) What Index ID?

3) Restarting the client "fixes" the problem (it will eventually happen again, but it stays fixed for 100's of thousands of transactions before the error happens again); if there is index corruption on the server, why would restarting the client have any effect?

Tnx!