Well I have a hunch…
All of your main javascript is being loaded by ‘flywheel’. Is that your hosting company? This includes the forum scripts.
The problem – I believe – is that as part of the plupload code, a file named plupload.full.min.js is being loaded from the SP plugin as it should be.
But a little later in the page load a second copy is being loaded from the WordPress includes file. This should not happen. And to compound the problem, it is an older version so we now have conflicting code.
SO – quick explanation. This code is used by WordPress in the media library system that allows you to upload images for your blog posts and pages. It needs to be loaded n the WP Admin. But your ‘Flywheel’ is loading it in the public side as well.
This also explains why the problem only exists for WP Admins as only people who can use the Media Library in the admin need the code to be loaded. But again – in the back end admin side.
I am not, personally, a fan of scripts being loaded by third party services and from third party servers as they can cause endless problems (like this one of I am right). But if you really feel you need to use this service then it is vitally important to ensure that only – and all – the correct scripts are being loaded.