Which version/os are you running? I just tried it in 65.0a1 (2018-12-02) (64-bit) (Linux), and it works just fine for me. Have you got other extensions that might interfere somehow? Are there any messages in the console?

Another user mentioned a similar problem. Turns out he enabled blocking all third-party cookies. Apparently this also breaks extensions. The solution is to add adventofcode.com as an exception to always allow cookies and site data.