What image are you using? If you change the shell in the top right hand corner does it work? I’ll take a look, see if I can fix it.
LXDMosaic handles the proxying itself so it can do advanced things like record sessions and control the timeout. Others seem to be happy leaving the proxying up to HaProxy / Nginx (at a cost of loosing some advanced features they could implement).
If you just want a simple PHP script to get you started I wrote one here (you should be able to adapt it to your language).