Log in attempt fails without any error messeges

when i try to log in with user name and pass word it come back to log screen without display any error messages. then i have to go to the forget passsword option and enter my email . after that i can log in with my old password and username ( without reset the password)
Further in my Widget section i can drag or move any widgets it all are locked. site work perfectly well when i disable the s2member plugin.

please help me on this to sort it out
Thank you

https://wordpress.org/plugins/s2member/