MathJax 4.0.0-beta.3~4 does not have es5/ folder but the version before 4.0.0-alpha.1 has it. That caused the error.
I changed the config with es5 directory in this PR.
But 4.x is still a beta version and there is no stable 4.x release yet, so I'm not sure we should do the update now.
Please merge as appropriate, thanks
Should fix #1669, #1694
As described in #1694
But 4.x is still a beta version and there is no stable 4.x release yet, so I'm not sure we should do the update now. Please merge as appropriate, thanks