Reverse Proxy Issue - Synchrony doesn't work

<VirtualHost *:443>
  ServerAdmin webmaster@testconfluence.bitvoodoo.ch
  ServerName testconfluence.bitvoodoo.ch
  ServerAlias www.testconfluence.bitvoodoo.ch
  ServerAlias testconfluence.bitvoodoo.ch.bitvoodoo14.nine.ch

  # Do not index Testdomain
  SetEnvIf Host "testconfluence.bitvoodoo.ch.bitvoodoo14.nine.ch" TEST_URL
  Header set X-Robots-Tag "noindex, nofollow" env=TEST_URL

  DocumentRoot /home/www-data/testconfluence.bitvoodoo.ch

  ErrorLog /home/www-data/logs/testconfluence.bitvoodoo.ch.error.log
  CustomLog /home/www-data/logs/testconfluence.bitvoodoo.ch.access.log combined

  UseCanonicalName off

  # SSL
  SSLEngine on
  SSLCertificateFile /etc/ssl/letsencrypt/certs/testconfluence.bitvoodoo.ch/cert.pem
  SSLCertificateChainFile /etc/ssl/letsencrypt/certs/testconfluence.bitvoodoo.ch/chain.pem
  SSLCertificateKeyFile /etc/ssl/letsencrypt/private/testconfluence.bitvoodoo.ch/priv.pem

  Alias /nine-ssl-check "/var/www"

  # Directives
  # Jira Proxy
  ProxyRequests Off
  ProxyPreserveHost On
  
  <Proxy *>
    Order deny,allow
    Allow from all
  </Proxy>
  
  ProxyPass /nine-ssl-check/ !
  ProxyPass /.well-known/ !
  ProxyPass / http://localhost:8090/
  ProxyPassReverse / http://localhost:8090/
  # AWStats
  Alias /awstats-icon "/usr/share/awstats/icon/"
  Alias /stats "/usr/lib/cgi-bin/awstats.pl"
  <Location /stats>
    Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
    AddHandler cgi-script .pl
    Order allow,deny
    Allow from all

    SetEnv AWSTATS_FORCE_CONFIG testconfluence.bitvoodoo.ch

    AuthName "Stats"
    AuthType Basic
    AuthUserFile /home/www-data/.htpasswd
    require user nine stats stats_www-data stats_testconfluence.bitvoodoo.ch
  </Location>
</VirtualHost>