I have been implementing WPS Hide Login plugin in order to block wp-admin brute force attacks.
In the process I have come to realise that WP Main is relying on wp-admin URL for access, or at least that is what I think it is doing, as all the sites with the modified wp-admin URL have stopped connecting.
Hmmm, its not WPS Hide Login thats the issue. Still looking for the cause, but I have just tested against 3 sites and got slightly different results. Will test again.
Tony