Cannot create a user thread error

I have a large number of normally well behaved (with Sql Server and Oracle) unit tests failing when connected to my IDS test databases. The error message is "[HY000] [Informix .NET provider] [Informix] Cannot create a user thread". (This is with Visual Studio 2008 Team System, using MSTest as the test framework, if that means anything.) This is overwhelmingly the single largest cause of failing tests in the suite. Can anyone suggest the most likely cause of this? It only occurs when the tests are pointed at the IDS instance. The tests (as far as I can tell by inspection) are quite diligent about creating connections in a 'using' clause so that they are always closed after use, so I am assuming that this is not an issue with too many connections being open, but I may be wrong on that conclusion. TIA Neil.