I am using Membership provider in my webapp and I have this problem too.
I was able to login last time. but now this message appears:
Your login attempt was not successful. Please try again.

when I tried to login in Firefox I logged in successfully!!
It seems to be a problem with IE.
I did following  jobs to solve the problem but they weren't  usefull.
- I deleted offline, cookies and history of IE.
- restart IIS.
- reboot my computer!

 strange case, please help...