Putting this answer here so I can include links.
In the Mendix Cloud, you can access your application logs. Directions here: https://docs.mendix.com/developerportal/operate/logs
You could also search for the text in the error message in your app in Studio Pro. In the Edit Menu, select Find (or use Control + F if you’re a fan of keyboard shortcuts). They type in some part of that message and see if you get any results. This would give you a starting point for figuring out what is causing the problem.