Can't install

Well, some progress. But problems persist.

After changing the permission on the installer.class.php file, I got it installed. However, although I uninstalled OpenSEF, cleaned the cache and reinstalled SEF Advance several times, my website is not functional and the browser displays this message:

We are really sorry, but our sef_ext.php file is provided only for the OpenSEF component. You can install the OpenSEF component, remove the sef_ext.php file or write the sef_ext.php for the SEF component which do You use

When I checked this file in the SEF Advance folder, it is the one for SEF Advance. Nothing to do with OpenSEF. And I can't find any other such file.

It is SEF extension for some component that you have installed. Maybe sobi2?
Go through all components you have in ./components/ directory and delete / rename files called sef_ext.php or replace them with their SEF Advance equivalent found on the extensions site .

Thanks. SOBI2 was the one. It seems fine now.