Group Policy Restricted Groups - The following accounts could not be validated

I'm trying to modify a GPO to update the restricted groups on our servers. 

I have 2 groups, one is added to the GPO with no issues, the other one give me the following error:"The following accounts could not be validated"

I've tried adding the group from the locally installed RSAT tools on a win7 machine and also from a 2008 R2 DC. I get the same result.

I can't find much anywhere online to help. Any ideas?