M-G, did you happen to apply security updates before this problem began? As Devon said, we are seeing the same issue and the only change we've made recently is to apply security updates.
My browser is showing 404 errors for some of the JSAPI4 items, which I am guessing is causing the problem. The files are on the sever, but the URL to them is throwing a 404. Are you seeing this too?
GISP