Upon checking the server requirement url,
it prompts one issue:
Although NOT required, s2Member recommends that your WordPress installation be configured with a matching HOST name. This can be changed in the Dashboard, under: WordPress -› Settings -› General -› WordPress/Site URLs. Your current configuration does NOT match: domain.com
I changed wordpress url before from the database general settings,
from domain.com to www.domain.com
Currently the general setting url is www.domain.com
Not sure how to solve this issue now?