Check the settings under
WP Admin -> s2Member -> Restriction Options -> Unique IP Access Restrictions, and also check under
WP Admin -> s2Member -> Restriction Options -> Brute Force IP/Login Restrictions.
Configure everything there, I don't recommend you disable the restrictions though, just configure it correctly.
Then, find the user which is getting the errors (you can do that under
WP Admin -> Users -> All Users), click edit, in the s2Member section of the user info, find the fields that appear in the image below:
You can see the IP Adress of the user at the time of registration, and you can also reset the IP Restrictions on that user, which is the recommended way to handle this, instead of removing the restrictions altogether.
Hope this helps.