AOVPN - RAS Server - Specified interface was not present in MGM

Morning All

I have two RAS servers for Windows AOVPN.

Over the last few days i've spotted the following error in the event logs and customers have been reporting failed connections. 

"Specified interface was not present in MGM"

I really can't find anything useful online in regards to this error to point me in the right direction. 

Can anyone help?

Thanks