If you look at the line above, it tried to load first.js but failed with a 500 error (internal server error). If you try loading that first.js URL in your browser, you may see an error message explaining why, otherwise you'll have to do some more investigation in your web server logs.
We have a similar issue. At random, some navigation buttons are not clickable, and course content does not display. The issue seems to come-and-go, but even when navigation does work correctly, the JS error remains.
