I installed the latest version of JUX Mega Menu on Joomla 3.8.12 and Virtuemart version 3.2.15 and it worked fine for a few days. but it stopped working suddenly after a few days.
I changed the menu to prevent showing product items (there are many thousands). Then in the admin area I got the error: 0 Error decoding JSON data: Syntax error.
I uninstalled the module then re-installed and could then access the Menu settings in administrator again. But, once again, when I save and close, the same error appeared.