Installation failed : error: "Table Prefix" must not be empty.

Hi everyone !

I’m having issues installing a WordPress 4.1 on a shared host. There is already several wordpress installations on it with different prefixes for the database.
Never had any issues.

This time :

First step of the installation, I fill correctly all the fields and I got an “error: “Table Prefix” must not be empty.”. Table prefix is not empty but still, impossible to generate wp-config.php this way.

I think “no problemo” and I upload via FTP the wp-config.php with all the information needed, including the wp_prefix.

I got to step 2 and I fill all the fields again. This time I got a “Please put a valid username” error. I don’t know what’s going on.

I’ve checked with my host provider and they think there is no problems on the server side.

I know some people have had the same issues but I can’t find any solutions that work for me :
No php.ini on my server
No .htaccess on the root either.
My 3 other wordpress are in another folder but I’v already installed wordpress in this one so I don’t think the location matter in this case.

Any one have a good advice for me ?
It would be deeply appreciated 😀

Thanks in advance,
Johann