When saving a unit, CoursePress gives error

I've got a problem with the CoursePress Pro plugin. When I want to edit or remove a unit, there's an error. The message I get when I want to save whole units is "Something went wrong. The unit was not saved!"

I have tried the following:
I've deactivated all plugins except the CoursePress Pro plugin and the WPMU Dev dashboard, but it still shows the error message. Also I've tried to use another theme, to make sure it's not a conflict with the theme, but this also is not the solution.

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