I have the same problem. It is really annoying.
I’ve discovered a workaround. There should be a registry key called:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\EndNote12.AddinServer]
@=“EndNote12.AddinServer”
[HKEY_CLASSES_ROOT\EndNote12.AddinServer\CLSID]
@="{575B6FE9-477B-4595-A478-9B141A98D869}"
On computers where this exists the ‘insert citation’ works, on ones where it doesn’t the ‘invalid class tring’ happens. I discovered this from using Sysinternals Process Monitor and watching for errors.
When I open Endnote X2 this key is deleted! I re-merged a reg file and again when I opened Enx2 the key is deleted, so I have had to merge it then add a custom advanced deny delete permission for the loal machine group everyone on that key in the registry.
I now have to work out a way of globally merging and applying the deny permission for all PCs on my network.
Hope that helps .