Why are my users seeing “Too many IP addresses accessing one secure area”?
This is caused by Unique IP Access Restrictions.
See: WordPress Dashboard → s2Member → Restriction Options → Unique IP Access Restrictions
If you're running the pro version you can avoid this altogether by using Simultaneous Login Monitoring instead of Unique IP Restrictions. See: WordPress Dashboard → s2Member → Restriction Options → Simultaneous Login Restrictions for details.
However, it's still good to keep Unique IP Restrictions enabled to some degree if you're using Specific Post/Page Access, as that helps to protect that form of access also. So for instance, if I were running the pro version of s2Member I might set Unique IP Restrictions to around
50 (so it's on), but only for really abusive behavior, and then use SLM (Simultaneous Login Monitoring) to be more strict about how many people can access a single account at the same time.
See: WordPress Dashboard → s2Member → Restriction Options → Simultaneous Login Restrictions
Simultaneous Login Restrictions requires s2Member Pro.