>>I don't think that any SQL Server setting can resolve the problem because it's on .NET side. Google on '.net Failed to enable constraints'
>
>When I did that search on Google, because this is so generic, I ended up with way too much situations here and none of those I have seen seem to relate to the situation I had. The only think I could see if that, if on the .NET side, the VFPOleDB provider would allow that SQL to be executed but not the SQLClient provider.

Did you see this http://weblogs.asp.net/rosherove/archive/2004/10/03/DataSet-hell-_2D00_-_2200_Failed-to-enable-constraints.-One-or-more-rows--contain-values_2E002E002E002E002200_.aspx

In particular, it sounds like you may try to set .EnforceConstraints to false according to this message.

Read also through all the comments if the above would not resolve the issue. There are several other useful suggestions in this blog.