Enable "Debug" in HTML library Manager through configMgr, then the form editor report lots of css and JS errors for 49 times:
Refused to apply style from 'http://localhost:4502/var/clientlibs/libs/granite/ui/clientlibs/quickactions/less/generated/quickaction-default.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
I think a better approach to enable debug for individual files will be to add the query string parameter “debugClientLibs=true” to the URL in the address bar, before any URL anchor. For example http://localhost:4502/siteadmin?debugClientLibs=true#/content/YourSite/...
This option causes the client library to load all of its files separately instead of consolidating them, with just one drawback- enabling debug mode of the client libs can break the page but still can help you troubleshoot.