Existing User Can NOT log in again

[CorreoHS]

FO:
When a new user enter information the this user still log in, if the new user log out, the user become in existing user.
this existing user can not log in again,
user receive this message " The connection was reset The connection to the server was reset while the page was loading. "

Any idea how or where to look at to resolve this issue?

this is an issue just with exsiting user,

FO : New User works great
BO: Admin, yes ,they can log in.

I love PrestaShop! its a great! script

Thanks