iframe not visible when inside a post tab

Howdy Plugin developer!
In my test blog site mentioned above I use a plugin called wptab – WordPress Post Tabs. On one of the Tabs (No.5) I want to show an iframe showing a swiss map with a gpx track. This does not work. I tried both calling the iframe directly and with “Advanced iframe”. Neither works.
In my production site the same plugin / iframe combination works perfectly: https://chregublog.marba.ch/geschinerstock/ – Tab swisstopo.
In my test site I use a different wordpress theme called Rowling. I would like to move my production site to the new theme “Rowling”. However, without the iframed Tab I am not going to switch. I tried several other Tab Plugins (there are a lot!), but only “wptab” also works with google maps.
Strange: Hitting the Ctrl-Key on the Keyboard and scroll the wheel on the mouse let the iframe appear.
Any idea on what to try or do? Thanks a lot for helping.

The page I need help with: [log in to see the link]