Sync only works one way (Unauthorized access – Invalid remote IP address)

Hi,

I have two sites that need their users synced. The sync works perfectly from Site A -> Site B, but, when I test login from Site B -> Site A, it throws the following error :

The remote website encountered the following error: Unauthorized access (invalid remote IP address)

All the incoming and outgoing actions work perfectly fine on Site A, but, all the incoming and outgoing actions throw the above mentioned error on Site B.

Please suggest any workaround for this issue.