Pdelcast, thank you so much for the advice. It works now
Prior to that, I actually tried to remove all Castle-Link entries in Windows registry, including deletion of the oem29.inf, cygf32.sys, cyglib.sys, etc but somehow it didn't work. There seem to be some duplicate copies of the driver messed up somewhere in the registry database / backup, but access is denied.
So I decided to install on a different directory instead, and then it works.
I hope the engineers can solve this problem in the next Castle-Link release (and if possible clear up the previous mess left over in the registry).
Thanks.