I would also like to add one thing that may not be immediately obvious: If you are installing a license that was issued for version 5.20, then if you are using it with version 5.20, in the License Manager, the "Component" selection should be "QuickOPC.NET 5.2".

You can use the same license for version 5.21, but because in this version, the earlier product editions (.NET, -COM, -UA) are merged together, you need to install this license under Component = "QuickOPC 5.2" (this comes up automatically as one of the options when version 5.21 is installed using its setup program).

I hope this helps.

Have you followed all instructions in the "Concepts" document (available from Start menu after QuickOPC is installed), Section 8. Application Deployment?