I saw your previous posts before, seems like your problem mainly because 'something' was preventing the page being loaded. I had something similar problem with one of my clients. This solution might not work for you.
The other indication is the server is very slow. Try to turn OFF all security features that is ON (where QuickQR is hosted), test it. if the error 500 still persist then you can turn all ON again, but if the error 500 is gone, then enable one-by-one that security feature, test it after you ON one-by-one, this is to find out which security feature blocks the app from loading the page properly.
if you are using cpanel hosting, go to security menu.