Check your global catalog server(s) in that site. I had two Exchange servers that showed this error at the same time. I saw that the GC was having some Kerberos errors and then rebooted during that time frame.
I'm guessing there are multiple reasons to have this error, but it's probably because that service can't enumerate group membership at that specific time. I'm also guessing that service is periodically trying to check and and succeeding and thus no noticeable interruption. Just a theory.
