Hi Kovid,
Firstly I think that you don't have anything to do with this as you are not the one to maintain Calibre on Archlinux, but I report anyway in case it happens with all Linux distros.
The attached epub works with Calibre Editor, Calibre Viewer and Calibre Content Server on Windows 10. No problem at all. But the Content Server on Archlinux(arm) says it "Failed to download some MathJax data"
Code:
output/chtml/fonts/woff-v2/MathJax_Size1-Regular.woff
Failed to communicate with "/mathjax/output/chtml/fonts/woff-v2/MathJax_Size1-Regular.woff?1594872528521", with status: [404 (error)] Not Found
input/asciimath.js
Failed to communicate with "/mathjax/input/asciimath.js?1594872528522", with status: [404 (error)] Not Found
input/mml.js
Failed to communicate with "/mathjax/input/mml.js?1594872528520", with status: [404 (error)] Not Found
(and many more)