Can't modify Header

[WiltshireAirlines]

FIXED: PLEASE IGNORE THIS THREAD, I HAD BLANKSPACE BEFORE THE OPENING PHP TAG

I know this question has been asked before on this forum but the solution presented there does not work for me. When I click the log in button, after filling out my email & pass correctly, I receive this error:

Warning: Cannot modify header information - headers already sent by (output started at /home/vaal/public_html/core/modules/Login/Login.php:1) in /home/vaal/public_html/core/modules/Login/Login.php on line 136

I've looked up this problem and it seems that it's caused by sending output after headers are already sent (which I don't really understand what headers are), but I have no idea how to fix it...I'm no PHP guy.

Thanks in advance for the help!

Let me also add on this information: this error only started occurring after I transferred the whole site to a new domain. Also, I do see this line of code that looks pretty suspicious:

header('Location: '.url('/'.$this->post->redir));

This is line 136 of Login.php, which is where the error is supposedly coming from.

When I click Logout, I get these two errors:

Warning: Cannot modify header information - headers already sent by (output started at /home/vaal/public_html/core/modules/Login/Login.php:1) in /home/vaal/public_html/core/common/Auth.class.php on line 341

Warning: Cannot modify header information - headers already sent by (output started at /home/vaal/public_html/core/modules/Login/Login.php:1) in /home/vaal/public_html/core/modules/Logout/Logout.php on line 25