Running a sub domain multisite I can't access the network sites screen (/wp-admin/network/sites.php) the screen is blank when it loads. I deactivated all plugins and began reactivating, the network sites screen could be accessed up until I reactived Defender Pro.
I checked another multisite (with subdirectories) and got a php error when trying to view the network sites page which was refering to a slow or no php response and directing me to the php error log files where I noticed references to 'Failed to read FastCGI header' and an 'error dispatching request to …….. /admin.php?page=wdf-advanced-tools&view=mask-login.
When I deactivated the masked login feature on defender Pro I could then access the network sites page on both multisites.
Problem returns on re-activating the masked login feature.