IJulia Syntax Highlighting

It happens regularly that I open a notebook and syntax highlighting is broken. Next day it might work again - or not. Sometimes another notebooks started from the same server front page will work fine. I believe this is related to the following warning message:

[W 11:40:40.442 NotebookApp] 404 GET /components/codemirror/mode/julia/julia.js?v=20170501113954 (::1) 1.30ms referer=http://localhost:8888/notebooks/Silicon-Dipole.ipynb

Does anybody have an idea what causes this, and how to prevent it?