After migrating AD from 2003 to 2012, I ran into this issue on a RADIUS server running 2008 R2, used for authentication. When attempting to retrieve AD information for a particular user, the following error appeared:

Error
An error (1301) occurred while enumerating the groups. The group's SID could not be resolved.

The solution (at least for Windows 7 and 2008 R2), is to install hotfix 2830145.

The hotfix is only available by request through e-mail.

When attempting to log on to a 2012-based domain controller, the following SIDs are unmappable:

S-1-18-1 : Authentication authority asserted identity

S-1-18-2 : Service asserted identity

2012 introduces two new securit  ...

Running OpenVPN on Vista, works, but requires some workarounds.
Current list of issues:

• connection gets established, but route addition fails with route addition failed usingCreateIpForwardEntry

Read further for the solutions...

connection gets established, but route addition fails with route addition failed usingCreateIpForwardEntry

This problem always occurs on a Vista. What happens is that while the connection does get established, the routes are not being added properly, because of an API change in Vista.
To circumvent the issue, add this line to your OpenVPN config file:

route-method exe

This way, the route addition is called directly using the commandline utility.