Hello Alan,
Off the top of my head it sounds like your page contains some custom code somewhere that is making the web modeler sad.
You can do a quick canary-test by opening the project in the web modeler with the developer tools console open (F12 → Console). If you have a bug in your custom JavaScript or CSS, the console should start regurgitating red letters as soon as the page loads. Hopefully the output will point you/your designer/your custom widget developer towards fixing the incompatibility.
Have you synchronized your modeller with the webmodeller?