error 500

Please remove the old theme and install the new one from WP.org.
To see the error text open wp-config.php and set WP_DEBUG to True and tell me which error are you getting.

Yes, sorry that I didn’t do that earlier.
Error:
Fatal error: Can’t use function return value in write context in /var/www/userhome/harris/sites/xxx.xx/secure-www/wp-content/themes/realistic/functions.php on line 432

Edit:
This also on the front end

Warning: include(functions/widget-fblikebox.php): failed to open stream: No such file or directory in /var/www/userhome/harris/sites/xxx.xx/secure-www/wp-content/themes/realistic/functions.php on line 156

Warning: include(): Failed opening ‘functions/widget-fblikebox.php’ for inclusion (include_path=’.:/usr/share/php:/usr/share/pear’) in /var/www/userhome/harris/sites/xxx.xx/secure-www/wp-content/themes/realistic/functions.php on line 156

For now, please do the following steps to fix the issue until the next version is released:
open functions.php (line 432) and change this:

if ( !empty( realistic_entry_tags() ) ) {
	echo realistic_entry_tags();
} ?>

To this:

echo realistic_entry_tags(); ?>

Next, remove this line (line 156):

include("functions/widget-widget-fblikebox.php");

I doesn’t exist in the functions.php file of 1.1.1 version. I don’t know why you still have that line?? I you are using an old version please update to version 1.1.1 then clear the cache (If you are using a caching plugin like WP Super Cache)