Thank you, I solved it by removing session_start(); from the start of file. It works! And yes, home page was loading but login just didn't work.

Did you removed session_start() from the start of flile? also please follow the instructions mentioned by ~Z~ Man.

What about your site's homepage - is it loading?

It's difficult to help without knowing any more details. Please follow the instructions in
https://www.opensource-socialnetwork.org/wiki/view/1101/how-to-report-a-bug-error

Hi Arsalan, i did what you say but problem is still the same.
see what i did here: https://gph.is/2zny9me
Thank you for help.

Hi Zoran, the issues seems with your sessions, edit the file system/start.php and place session_start() after line 34. So the file may look like :

....
...
foreach ($Ossn->classes as $class) {
    if (!include_once(ossn_route()->classes . "Ossn{$class}.php")) {
        throw new exception('Cannot include all classes');
    }
}
session_start();
foreach ($Ossn->libraries as $lib) {
    if (!include_once(ossn_route()->libs . "ossn.lib.{$lib}.php")) {
        throw new exception('Cannot include all libraries');
    }
}
...
...

See demo here:

https://gph.is/2xF6gEn