Issues using Apache as a HTTPS proxy: 403

[RedstoneWizard08]

Hello! I am trying to use Kasm behind an Apache HTTPD reverse proxy. When I try to connect to a workspace, it fails with 403 in the DevTools console. Can you help me? Here is my Apache config:

<VirtualHost *:443>
        ServerName kasm.xxxxx.xxx

        ServerAdmin webmaster@localhost
        DocumentRoot /var/www/html/public_html

        ErrorLog ${APACHE_LOG_DIR}/error.log
        CustomLog ${APACHE_LOG_DIR}/access.log combined

SSLProxyEngine On
    SSLProxyCheckPeerCN off
    SSLProxyCheckPeerExpire off
    RemoteIPHeader X-Forwarded-For
    RemoteIPHeader X-Real-IP
    Header set X-Forwarded-Proto https

        RewriteEngine On
    RewriteCond %{HTTP:CONNECTION} Upgrade [NC]
    RewriteCond %{HTTP:Upgrade} =websocket [NC]
    RewriteRule /(.*)           wss://localhost:4937/$1 [P,L]
    RewriteCond %{HTTP:Upgrade} !=websocket [NC]
    RewriteRule /(.*)           https://localhost:4937/$1 [P,L]
    ProxyPassReverse /          https://localhost:4937/

SSLCertificateFile /etc/letsencrypt/live/kasm.xxxxx.xxx/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/kasm.xxxxx.xxx/privkey.pem
Include /etc/letsencrypt/options-ssl-apache.conf
</VirtualHost>
<VirtualHost *:80>
        ServerName kasm.xxxxx.xxx
        DocumentRoot /var/www/html/public_html
        RewriteEngine on
        RewriteCond %{SERVER_NAME} =kasm.xxxxx.xxx
        RewriteRule ^ https://%{SERVER_NAME}%{REQUEST_URI} [END,NE,R=permanent]
</VirtualHost>

# vim: syntax=apache ts=4 sw=4 sts=4 sr noet