Warning php messages when setting instructor permission

Hello!
I'm receiving some alerts when I try to modify instructor permissions from CoursePress Pro>Settings, like this one:

Warning: Invalid argument supplied for foreach() in /web/htdocs/www.******.it/home/wp-content/plugins/coursepress/includes/admin-pages/settings-instructor-capabilities.php on line 25
Same messages with the difference at the end of the string... on line 30, on line 29, etc.

(Sorry for censored marks on the Domain name. Obviously I have read and made all the tips you suggest in "before you post".)

Thank you
David