Hi @john3, subfolder installation is painful 
I compared my config (left) and yours (right), take a look at line 12, probably you forgot to change word forum. Next, I have extra block replace, lines 22-29:
The end:
External proxy:
location /discuss {
## This is ProxyPassReverse for Nginx
proxy_set_header X-Forwarded-Host $host;
proxy_set_header X-Forwarded-Server $host;
proxy_set_header X-Forwarded-For $remote_addr;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_read_timeout 3600;
proxy_connect_timeout 240;
proxy_set_header Host $host;
proxy_set_header X-RealIP $remote_addr; #Important line for our logging objective
## End This is ProxyPassReverse for Nginx
proxy_pass http://192.168.1.2$request_uri;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;