Server.php -> openIPCSocket() -> socket_bind -> creates a file in /tmp but www-data has no permissions to use it. This happened when i tried to send IPC data from an action of a website to the WebSocket server. This change did the trick:
FROM
if (socket_bind($this->icpSocket, $ipcSocketPath) === false) {
throw new \RuntimeException('Could not bind to ipc socket.');
}
TO
if (socket_bind($this->icpSocket, $ipcSocketPath) === false) {
throw new \RuntimeException('Could not bind to ipc socket.');
} else {
chown($ipcSocketPath, 'www-data');
}