Fatal error: Call to undefined function is_user_logged_in()

My multisite has been working fine for the pas few days, but now it returns this error each time I try to open one of the subsites: Fatal error: Call to undefined function is_user_logged_in() in /home/xxxxxxxx/public_html/wp-content/plugins/pro-sites/pro-sites.php on line 1799

When I disable Pro Sites I'm able to load the subsites.

I searched your archives but couldn't find any references to this particular error.

I also posted this through our dashboard and have enabled support access.

Any thoughts on how to resolve this one?

Thanks.

Bob