I'm not familiar with apache but the text page issue is due to the resource like css got wrong address after been passed,you can open access log or use browser's web debug tools to see what's the problem with your rule.

and another workaround is if you can't work out with reverse proxy rules, maybe you can work on something inside that jar file like change the directory in admin/admin.html and other files.

my reverse proxy prefix setting:

  "reverseProxyPrefix" : "comic"

I am able to do it with nginx and my rules are 

location /comic {
proxy_pass http://localhost:2202/comic;
}
location /comic/admin {
proxy_pass http://localhost:2203/comic/admin;
}