Problem with user login

Hello,
I have a strange problem.
When a user want to login, after entering his email and password, he got a Warning: No match for E-Mail Address and/or Password error.
When he then enters the same email and password, he is logged in.
I checked al kind of logging, but can't find the problem. (No errors logged!!!)
Can someone help me??

Thank you

Henk

I forgot. I am using Opencart 1.5.6.1