[TablePress] Import fails all of a sudden if contains embed/iframe

Hello,
working with WPMU environment and having TablePress installed, try to create tables with small iframes/YouTube embeds per row.

Import worked fine, in the beginning (a couple of hours ago). I tried a blank YT-link to use core functionality, which did not work. [embed] did not render the iframe either. The [iframe] plugin seemed to work randomly, I postponed the analysis.

Now, some hours later, I found out that the YouTube Link was wrong. Seems I need the http://youtube.com/embed/XXX instead of http://youtube.com/watch?v=XXX

So far so good. Just in the moment after the first test was working, the import stopped at all working with any YT-links. On clicking the “Import” button, I get a window attempting to open/download admin-post.php

It’s a bit like this issue, but without 404: https://wordpress.org/support/topic/import-stopped-working/ – however there was no solution provided.

I checked PHP log, no errors available.
I disabled Firewall/Wordfence completely, no change
I removed [embed] or [iframe] -> it works!
I changed YT URL with Vimeo -> fail

What gives me the willies is that the import worked some hours ago *WITH* several [iframe]/[embed] and I did not touch anything in the meantime.

What else could I do to to dig into this? I’m out of ideas…
Thanks for any help.
ellaT