location / {
    proxy_pass https://kasmweb:6901;
    proxy_set_header Authorization "Basic a2FzbV91c2VyOnBhc3N3b3Jk";
    proxy_pass_header Authorization;
    proxy_set_header Host $host;
    proxy_set_header X-Forwarded-Scheme $scheme;
    proxy_set_header X-Forwarded-Proto  $scheme;
    proxy_set_header X-Forwarded-For    $remote_addr;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection $http_connection;
    proxy_http_version 1.1;
}

Thanks to this discussion, this worked for Nginx proxy Manager and it connects imediatly. including this in authentik configurations breaks it tho