width of the iframe set to 0

Hi Anders
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 map with a gpx track. This does not work. Some script of the Rowling Theme seems to set the width of the iframe to 0 (zero).
In my production site the same plugin / iframe combination works perfectly: https://chregublog.marba.ch/geschinerstock/ – Tab swisstopo.
I would like to move my production site to the new theme “Rowling” ’cause I like it very much. 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.

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