In order to solve this issue you should check the following:
Please make sure that permissions for the following folders are set to 775 (drwxrwxr-x):
Following files should have permissions set to 664 (-rw-rw-r--):
Also, following files should have permissions set to 755 (-rwxr-xr-x):
The rest of the files should have the permissions set to 644 (-rw-r--r--).
The rest of the folders should have the permissions set to 755 (drwxr-xr-x).
Possibly, your hosting provider is adding some script to your site's .php files which doesn't allow them to work correctly. You can try to add following directivies to your .htaccess file (should be located under website's root folder):
php_value auto_append_file none
php_value auto_prepend_file none
php_value short_open_tag 0