Unable to login/"Unknown error" on main screen

[Migsi]

After managing to setup everything on my raspberry pi (using php7 instead of 5), the main page won't let me log in. I am using the default password "1234" but it keeps telling me that is a wrong password. Also the cronjob wgetting the analogChart.php frequently, keeps telling me there was an internal server error while performing the task. Is all this because of php7 or am I doing something terribly wrong? Just in case that you wonder, there were no older packages for php in apt, otherwise I would have taken the ones you consider!