This question has been answered by multiple community members. See the first response.
OK, for anyone who hits the same problem, this was fixed by turning Friendly URLs off and then on again.
It's actually a problem with Google minify, the JS/CSS compressor used in the manager, and something we're working on for 2.2-pl. For now, you can just turn off CSS/JS compression and that will fix the issue (turn off compress_css and compress_js system settings).