Websocket (Ratchet/zeroMQ) with Apache proxy and SSL - on a Plesk server - apache

Question

I'm trying to get a websocket/push-server to run with SSL and can't get it to work.
Here are some informations about what I'm using and what I've done so far:
Ubuntu 16.04.6 LTS‬ server with Plesk 17.8.11
Apache version 2.4.18
Ratchet and zmq extension installed
autobahn.js for websocket connection
PHP-FPM (7.2.19) served by Apache
Firewall disabled for testing
Apache mods enabled: proxy, proxy_balacer, proxy_fcgi, proxy_http, proxy_wstunnel
When I use an non-SSL connection via autobahn.js to ws://my.domain.net:8888/ everything works perfectly fine.
As soon as I try to use wss://my.domain.net/wss/ I get 504 connection timeouts in my browser.
Part of my push-server.php:
$context = new React\ZMQ\Context($loop);
$pull = $context->getSocket(ZMQ::SOCKET_PULL);
$pull->bind('tcp://0.0.0.0:5555'); // set to 0.0.0.0 for testing, was 127.0.0.1 before but didn't work either
$pull->on('message', array($pusher, 'onMessage'));
$webSock = new React\Socket\Server('0.0.0.0:8888', $loop);
$webServer = new Ratchet\Server\IoServer(
new Ratchet\Http\HttpServer(
new Ratchet\WebSocket\WsServer(
new Ratchet\Wamp\WampServer(
$pusher
)
)
),
$webSock
);
$loop->run();
The server is listening on both ports:
php 3957 root 17u IPv4 823972456 0t0 TCP *:8888 (LISTEN)
ZMQbg/0 3957 3958 root 17u IPv4 823972456 0t0 TCP *:8888 (LISTEN)
ZMQbg/1 3957 3959 root 17u IPv4 823972456 0t0 TCP *:8888 (LISTEN)
php 3957 root 16u IPv4 823972455 0t0 TCP *:5555 (LISTEN)
ZMQbg/0 3957 3958 root 16u IPv4 823972455 0t0 TCP *:5555 (LISTEN)
ZMQbg/1 3957 3959 root 16u IPv4 823972455 0t0 TCP *:5555 (LISTEN)
My apache config:
SSLProxyEngine on
ProxyRequests Off
SetEnv proxy-initial-not-pooled 1
ProxyPass /wss/ https://my.domain.net:8888/
ProxyPassReverse /wss/ https://my.domain.net:8888/
I also tried to use
ProxyPass /wss/ wss://my.domain.net:8888/
but then I get the following error message
[proxy:warn] AH01144: No protocol handler was valid for the URL /wss/. If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule.
Now when I try to open a connection with autobahn.js to wss://my.domain.net/wss/ the request loads for a while and ends in a 504 Gateway Time-out and I get the following error message:
[proxy_http:error] (103)Software caused connection abort: [client x.x.x.x:33362] AH01102: error reading status line from remote server my.domain.net:8888
[proxy:error] [client x.x.x.x:33362] AH00898: Error reading from remote server returned by /wss/
Part of the Request Headers:
Cache-Control: no-cache
Connection: Upgrade
Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits
Sec-WebSocket-Key: WG/ajAsTaVhBgEeEz5wiUg==
Sec-WebSocket-Protocol: wamp
Sec-WebSocket-Version: 13
Upgrade: websocket
Part of the Response Headers:
Connection: keep-alive
Content-Length: 578
Content-Type: text/html
Server: nginx
Opening the https://my.domain.net/wss/ in a browser ends in an Gateway Time-out as well, served by nginx.
I've spent like 2 days already googling and trying different solutions but nothing seems to work.
If you need more information, please let me know.
Thanks in advance!