Thanks @felfert ! Sorry for using java instead of javascript.
I removed the proxy-lines in the httpd.conf, as it doesn’t work anyhow - for now…
Say, the problem/character of the javascript in the battery’s web interface can be determined somehow. Could one then use the ProxyPass command in such a way (using other built-in options?) that it could handle the web javasript based interface of the battery and run through httpd as originally intended? If this could be possible, I can, with the help from all experts here (!), try to assess the battery’s web interface locally, without hooking it up to the internet with all the risks that would imply.
Please let me know if this is a feasible way to go forward , if someone wants to take the challenge and to make the “rabbit hole” more open 
Some next steps would be:
Reload it in the browser.
Hit F-12 and check the Console for errors. You may have to click the Errors option to see them.
You can also click the Network tab and reload to see if the /main.js url is being retrieved or is being 404’d, although you might also see that error in the console.
Check these logs for 404s and errors:
/var/log/httpd/access_log
/var/log/httpd/error_log
There may be ssl versions of the above logs in the same folder.
Thanks @dannyw99 for your suggestions!
Quite busy though with other things at the moment. The battery problem got a lower priority right now. 
I’ll let you know what I can find!