Yes. There was write permission problem with ossndata directory. I had not setup group write permission to ossndata directory.

Now cache works.

Thanks.

With cache enabled, Ossn is creating a subdirectory named "cache" in your installation root directory for storing several javascript and css files there.

Since you already got a problem with the creation of the error_log file, it's likely that your webserver process has no write permission to your installation directory, hence creating of the cache directory will fail.

In case your site is completely unaccessable:
- Use your database to tool (PhpMyAdmin),
- load the ossn_site_settings record
- and change the values of both "cache" and "last_cache" to 0 (zero)

Your site should be accessable again now. (In doubt refresh your browser with CTRL-R)

Next make sure that your installation directory is owned by the webserver process (in most cases "www-data" and the permission is set to 755.
Then try to enable cache again.
In case it still fails, check the Ossn error_log file and your webserver's log file