Many thanks for the quick reply!
My appologies but I don't fully understand.
Until about 2 weeks ago (or so I think) everything was working fine. That means, the landing page plugin, the leads plugin and the standard column function in a page or post (and the content by country plugin as well).
We usually keep our installation up-to-date with any plugin updates and this is what we think happend. With a recent update (of the landing page and / or leads plugin) the problem appeared. Most pages using the column function got screewed up (and also the content by country plugin stopped working. Not sure if these are directly or indirectly related).
Yesterday, we turned off ALL (!) plugins and the (native) columns work fine.
If I turn back on ONLY the leads OR the landing page plugin, the columns no longer work.
(Same happens to the content by country plugin. On it's own, it works fine. When I activate also the landing page or leads plugin, it stops working. Once more, they ALL were working together in the past!).
So could you kindly elaborate by what you mean by "Can you update your theme/other plugin that is running 'column' shortcodes and prefix them?"? We have the Avada theme but my understanding was that the column function is (now?) native to WordPress?
Many thanks for your help and support!