openHAB with Apache Reverse Proxy: BasicUI SSE problem

This is openHAB 2.4.0 on Linux with Apache 2.4.

I managed to create a working SSL ReverseProxy for openHAB where almost everything works. This includes HabPanel with live updates (which is using SSE AFAIK), PaperUI and REST API docs.

But BasicUI SSE does not work. The problem is that it cannot get an SSE connection and then it opens more and more of them until openHAB is out of available SSE subscriptions.

This is my Apache config:

      ProxyPass /openhab
      ProxyPassReverse /openhab
      ProxyPass  /rest
      ProxyPass  /basicui
      ProxyPass  /icon
      ProxyPass  /chart
      ProxyPass  /static

      <Location /openhab/>
           allow from all
           AuthType basic
           AuthName "openHAB Proxy"
           AuthUserFile /etc/apache2/.htpasswd
           require user abc
           #ProxyHTMLInterp On
           #ProxyHTMLURLMap / /openhab/
           Header edit Location ^http: https:

As you can see, I already tried something with ProxyHTMLURLMap, but that did not work out. Therefore I have those ProxyPass lines for the sub folders.

Regarding BasicUI and SSE, I already tried adding this:

ProxyPass  /ws ws://      keepalive=on    retry=60
ProxyPassReverse /ws ws://        keepalive=on

But that did not help, either.

I am out of ideas. Anyone? Thanks!