You'll need to contact your web host. It looks like your server's PHP configuration is pretty restrictive.
You could try opening main.inc.php and commenting out the calls to ini_set (and ini_get). They're mostly in there for security purposes.
If your host allows you to use a custom php.ini file, you could set them in there, instead.
Regarding having to log in continuously, again, it sounds like a server configuration issue. This happens if the session save path isn't set correctly - you can get around this, but not with set_ini disabled... so, yeah... catch 22.
I'd work on correcting the ini_set errors first, though, and see if the login works after that.
Good luck with it. :)
