Update

html/js/streamplot.js

   url = url || [
     location.protocol == 'https:' ? 'wss://' : 'ws://',
     location.hostname,
-    location.protocol == 'https:' ? ':443' : ':80',
+    location.protocol == 'https:' ? ':' + (location.port || 443) : ':' + (location.port || 80),
     '/data/stream.ws'
   ].join('');
   var ctx = {
   var src = dataSrc([
     location.protocol == 'https:' ? 'wss://' : 'ws://',
     location.hostname,
-    location.protocol == 'https:' ? ':443' : ':80',
+    location.protocol == 'https:' ? ':' + (localtion.port || 443) : ':' + (location.port || 80),
     ctx.src.stream
   ].join(''));
 

service/websockify.service

 After=multi-user.target
 
 [Service]
-Type=forking
+Type=simple
 
 User=www-data
 Group=www-data
 Environment=TARGET_PORT=5900
 #EnvironmentFile=/etc/default/websockify
 
-PIDFile=/var/run/websockify.pid
 PermissionsStartOnly=true
 ExecStartPre=/bin/sh -c '\
 	touch /var/run/websockify.pid; \
 	chown www-data:www-data /var/run/websockify.pid'
 WorkingDirectory=/tmp
-ExecStart=/usr/bin/websockify --daemon --pid /var/run/websockify.pid \
+ExecStart=/usr/bin/websockify \
 	${LISTEN_HOST}:${LISTEN_PORT} \
 	${TARGET_HOST}:${TARGET_PORT}