Can't delete Network theme

Hey guys,

Getting a weird issue when trying to delete the Directory theme:

Get the messages below at the top of the screen and get told "Caution: This theme may be active on other sites in the network" - but its not!

Warning: fopen(/home/explorfw/public_html/wp-content/themes/default/style.css) [function.fopen]: failed to open stream: No such file or directory in /home/explorfw/public_html/wp-includes/functions.php on line 4339

Warning: fread(): supplied argument is not a valid stream resource in /home/explorfw/public_html/wp-includes/functions.php on line 4342

Warning: fclose(): supplied argument is not a valid stream resource in /home/explorfw/public_html/wp-includes/functions.php on line 4345