This one is driving me nuts. Post upgrade to 12.0.1, every page call is requesting a version of style.less that for some reason isn't available. It gives a 404 which then causes the browser to complain about getting an html (friendly 404) response instead of the CSS type it was expecting.
I've narrowed it down to the User - Banner Widget. Whenever I have that widget on the site in the header, the call to nowhere happens. With it gone, the page doesn't have any errors but obviously that doesn't work for site functionality. I've factory defaulted the widget (had very minor edits to restore the hover action on the username) and expired all of the caches and reloaded plug-ins and it still wants to request a path that doesn't exist. If the main site theme is in folder 123, the requested folder it's looking for is 456 (numbers for example only, if needed, can provide real numbers).
I suspect it is something with the widget's style.less wanting to be added after theme, but I have other widgets that are doing just fine.
What's the best way to track down where the path request is getting generated from the widget?