[Plugin: Tabbed Widgets] Width problem (tabbed widgets wider than sidebar)

Set up Tabbed Widgets on a test site, here https://173.83.225.185/test01_wp3/?p=125

Tabbed Widgets works great and looks good, thank you very much for this really useful add-on to the site.

I’m happpy with the look but I have one problem which prevents me from installing on my real site: https://iteractif.ch.

The width of Tabbed Widgets is wider than the site’s sidebar.

Looked at the CSS rendering in Firefox Inspect Elements and everything seems to be set to the 314px width of the sidebar.
So I can’t locate the source of the problem.

Could someone help me work it out, please.
Many thanks in advance.

https://www.ads-software.com/extend/plugins/tabbed-widgets/