Wierd screen blank after entering mysql info at installation; http500 internal server error after that

Hello
I tried to install pushtape, and i've got a problem with the installation.
At the database page, When i enter mysql DB, user and password, and hit the save button, The page refreshes and apparently the settings are saved, but i get the same page, but my fields are empty again.
If i write the info again, or if i don't, When i hit the save button the page becomes blank and nothing happens.

I try to restart the installation, and It guides me to the site info page. when i fill that info and hit save, i get http500 internal server error.

I'm using ubuntu 14.04 LTS

PHP Version 5.5.9-1ubuntu4.5
Apache/2.4.7 (Ubuntu)