The best answer is certainly to get away from ADODB in favor of the newer, but if that is in someway impractical, then you're going to have to do a bit of detective work. Database errors like this can be rather opaque in that the error message doesn't really tell you what the problem is in a very specific sense. Something isn't supported, maybe, but what? Is it just a typo, or is one of your arguments wrong? You don't get much information about that.

Unfortunately for you, since ADODB is so dated, few people will have much experience with it in .NET. But you DO have some experience with it, since, according to you, this code worked in a different program. So, what is different between the two? Because that's where your problem lies. Are both programs on the same computer (and if not, is one on a 64-bit while the other is not)? Are you using the same arguments? Something has to be different or else both would behave the same.