While this error occurs, the console in the modeler will show you the error as well. If you double click that error you'll get a message with a lot of detailed information, but also the name of the microflow or form or whatever is causing it.
Now you should be able to find out more about what is causing it.
Edit 2: I recently had an incorrect JSON as well. It was caused by the fact that I was trying to convert a non-existing item into another type. This can be avoided by adding the line
if $Entity/Object = empty then empty else (insert rest of the code here)
Thanks for the second edit!