Error | wpmu-plugin-manager

Hi,

Further to https://premium.wpmudev.org/forums/topic/possibility-of-private-themes-andor-plugins

Today we came across the following issue:
When we tried to activate 2 or more plugins together using the "Bulk Action" | "Apply" option, it gives blank page with "Fatal error: Call to undefined function deactivate_plugins() in /home/topsiteb/public_html/wp-content/mu-plugins/wpmu-plugin-manager.php on line 448"

* When back to the plugins list, the plugins chosen above seems to be active too.

What could be the issue?

Thanks & Regards