You made an update of multidomain plugin, and told it becomes a normal plugin, just put in /plugins directory.
But, after did that, i had this error message :
Please copy the sunrise.php to /var/www/web/wp-content/sunrise.php and uncomment the SUNRISE setting in the /var/www/web/wp-config.php file.
I read also the forum, and i saw several problem with this plugin update, maybe you must change the install tutorial, and explain clearly what we need to do.
Have a nice day,