Same issue - the ProxyPass works fine, but the problem i think might be caused by the Form itself after logging in.
This is the Output when i try connecting with my HTML Login Form:
Jun 12 07:23:02 scvelastic01.domain.local kibana[15240]: {"type":"response","@timestamp":"2018-06-12T05:23:02Z","tags":,"pid":15240,"method":"post","statusCode":404,"req":{"url":"/app/kibana","method":"post","headers":{"host":"kibana.domain.local","cache-control":"max-age=0","origin":"https://kibana.domain.local","upgrade-insecure-requests":"1","content-type":"application/x-www-form-urlencoded","user-agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.79 Safari/537.36","accept":"text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,/;q=0.8","referer":"https://kibana.domain.local/","accept-encoding":"gzip, deflate, br","accept-language":"en,de;q=0.9,en-GB;q=0.8,en-US;q=0.7,pl;q=0.6,da;q=0.5","x-forwarded-for":"172.16.51.128","x-forwarded-host":"kibana.domain.local","x-forwarded-server":"kibana.domain.local","connection":"Keep-Alive","content-length":"0"},"remoteAddress":"127.0.0.1","userAgent":"127.0.0.1","referer":"https://kibana.domain.local/"},"res":{"statusCode":404,"responseTime":8,"contentLength":9},"message":"POST /app/kibana 404 8ms - 9.0B"}
Jun 12 07:23:02 scvelastic01.domain.local kibana[15240]: {"type":"response","@timestamp":"2018-06-12T05:23:02Z","tags":,"pid":15240,"method":"get","statusCode":404,"req":{"url":"/favicon.ico","method":"get","headers":{"host":"kibana.domain.local","user-agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.79 Safari/537.36","accept":"image/webp,image/apng,image/,/*;q=0.8","referer":"https://kibana.domain.local/app/kibana","accept-encoding":"gzip, deflate, br","accept-language":"en,de;q=0.9,en-GB;q=0.8,en-US;q=0.7,pl;q=0.6,da;q=0.5","x-forwarded-for":"172.16.51.128","x-forwarded-host":"kibana.domain.local","x-forwarded-server":"kibana.domain.local","connection":"Keep-Alive"},"remoteAddress":"127.0.0.1","userAgent":"127.0.0.1","referer":"https://kibana.domain.local/app/kibana"},"res":{"statusCode":404,"responseTime":7,"contentLength":9},"message":"GET /favicon.ico 404 7ms - 9.0B"}
And this when i acces it without my form/authentication with the same URL (https://kibana.domain.local/app/kibana OR https://kibana.domain.local/)
Jun 12 07:23:35 scvelastic01.domain.local kibana[15240]: {"type":"response","@timestamp":"2018-06-12T05:23:35Z","tags":,"pid":15240,"method":"get","statusCode":200,"req":{"url":"/app/kibana","method":"get","headers":{"host":"kibana.domain.local","upgrade-insecure-requests":"1","user-agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.79 Safari/537.36","accept":"text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,/;q=0.8","accept-encoding":"gzip, deflate, br","accept-language":"en,de;q=0.9,en-GB;q=0.8,en-US;q=0.7,pl;q=0.6,da;q=0.5","x-forwarded-for":"172.16.51.128","x-forwarded-host":"kibana.domain.local","x-forwarded-server":"kibana.domain.local","connection":"Keep-Alive"},"remoteAddress":"127.0.0.1","userAgent":"127.0.0.1"},"res":{"statusCode":200,"responseTime":20,"contentLength":9},"message":"GET /app/kibana 200 20ms - 9.0B"}
Jun 12 07:23:35 scvelastic01.domain.local kibana[15240]: {"type":"response","@timestamp":"2018-06-12T05:23:35Z","tags":,"pid":15240,"method":"get","statusCode":304,"req":{"url":"/ui/favicons/favicon-32x32.png","method":"get","headers":{"host":"kibana.domain.local","user-agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.79 Safari/537.36","accept":"image/webp,image/apng,image/,/*;q=0.8","referer":"https://kibana.domain.local/app/kibana","accept-encoding":"gzip, deflate, br","accept-language":"en,de;q=0.9,en-GB;q=0.8,en-US;q=0.7,pl;q=0.6,da;q=0.5","if-none-match":""8e183c2e644fb050707d89402e1f7a120a95e4d2"","if-modified-since":"Thu, 12 Apr 2018 20:58:09 GMT","x-forwarded-for":"172.16.51.128","x-forwarded-host":"kibana.domain.local","x-forwarded-server":"kibana.domain.local","connection":"Keep-Alive"},"remoteAddress":"127.0.0.1","userAgent":"127.0.0.1","referer":"https://kibana.domain.local/app/kibana"},"res":{"statusCode":304,"responseTime":2,"contentLength":9},"message":"GET /ui/favicons/favicon-32x32.png 304 2ms - 9.0B"}