Urgent: Reverse Client Update to 6.5?!

Urgent!

We have two SAP servers for development. One server has only databases in version 6.20.36, the other one has only 6.5 databases.

Since I am bound to work with both versions of SAP Business One, I have tried to install a 6.5 client and a 6.2 client on my developing machine.

This definitely does not work. So I tried to reverse all and reinstall the SBO 6.2 client plus DIAPI and UIAPI.

When trying to connect via UIAPI I now receive the following error in .NET:

"System.InvalidCastException: QueryInterface for interface SAPbouiCOM.ISboGuiApi failed"

For some reason the references to the newly installed com objects do not work. Are there any hints on how to revert the installation of the 6.5 client?

Thank you in advance,

Lutz Morrien