[PROBLEM] Unable to open modal dialogs (configure, customize)

[Sander]

I am unable to open the modal dialogs for e.g. Configure and Customize in Firefox and Chrome.

The error that is logged in the browser console:

Code:

jquery-1.8.2.min.js:2 [Deprecation] Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the
end user's experience. For more help, check https://xhr.spec.whatwg.org/.

send @ jquery-1.8.2.min.js:2

This happened in 4.2 and also in 4.3. I can open the dialog in Safari though.