The obvious thing to check/rule out is missing files or incorrect permissions, though it could also be that static ascii files are being gzipped and that's tripping it up, or if there's a minifier plugin that does local requests on a misconfigured hostname, or a CDN using a subdomain that isn't proxied.
Manual HTTP requests through PuTTY and/or curl are a useful way to figure out what's actually going on.
Since you mentioned redirecting 80->443 I'm guessing the issue is/was with HTTP vs HTTPS and one of them was generating links that the other wouldn't serve.
ProxyRequests Off # reverse proxy to serverA/siteA 022718 <Location "/siteA"> ProxyPass http://serverA/siteA ProxyPassReverse http://serverA/siteA Order allow,deny Allow from all Authtype Basic Authname "Restricted Access" AuthUserFile /etc/apache2/.htpasswd Require user ****** </Location> # reverse proxy to serverB/siteB 051418 <Location "/siteB"> ProxyPass http://serverB/siteB ProxyPassReverse http://serverB/siteB Order allow,deny Allow from all Authtype Basic Authname "Restricted Access" AuthUserFile /etc/apache2/.htpasswd Require user ******* </Location>