[TYPO3-english] BE login doesn't work after install

[Barbara Wijbenga]

Hey all,

A new client of your wants to stay at her current hosting.
I looked at it and it seemed suitable for Typo3, but after installing, I 
coudn't login on the backend. ("Your login attempt did not succeed")
I'm not getting a PHP error or something.
I created multiple admin users in the install tool and I typed the passwords 
again and again, but I can't get in...
The users have been created in the database.

The only thing that's different from our hosting on the Basic Config page of 
the install tool is
Suhosin/Hardened PHP is loaded
suhosin.request.max_vars: 200
suhosin.post.max_vars: 200
But that can't be it, or can it?

Thanks,
Barbara