Tell us what you are trying to do? I made a page that is showing the parent courses where the visitor can make a choice from ("Opleiding"), then a query filter on "Locatie" and "Maand"
The thing is, it worked in the past, the parent courses where showing nicely, i suspect that maybe an update of one of the Toolset plugins broke it up?
Is there any documentation that you are following?
Is there a similar example that we can see?
What is the link to your site? The link to the testsite with the page: hidden link
You have to login to see it.
I added a screenshot with a debug message, not sure if it is related to the error in question....
I think we'll need access to your site to see the issue and how it is set up, so let me mark your next reply as private so that I can get log-in credentials from you.
You may want to create a temporary admin user for me to use that you can later delete. And be sure to have a current backup of your site.