My third post is deleting in all languages on website

The loss involves the complete deletion of 1 of my 3 posts on my website at https://3dresearchspecs.com/ , in all my 15 languages, including the main menu entry for the post and the metadata. The only things remaining for this post, the third post (post 3,) are the sidebar and footer widgets. In place of the post, where the post GIF used to be, this post GIF in German has been inserted https://3dresearchspecs.com/wp-content/uploads/2021/08/screenshot-38.png . The text “1” has been left intact on the post, the only text from the post that remains https://3dresearchspecs.com/wp-content/uploads/2021/08/screenshot-39.png. All the languages except the English language load the post deleted in this way. For the English language, in attempting to load the URL for post 3 into Google search, it loads a website, page not found, page https://3dresearchspecs.com/wp-content/uploads/2021/08/screenshot-37.png .

The deletion occurred after I had completed adding many extras for the German language, that is, I had created category pages, added in all the tags and hashtags in German, all done with German transliterated webpage URL slugs, added in breadcrumbs and category widget menus, added in many photo gallery widget entries and done the metadata. I estimate the deletion occurred between 0 and 17 hours after I finished this work.

I tried installing a backup of the website and it was successful for a time. I re-completed some of the work on the German language that the backup had not saved. Then I returned after a night break and the deletion had occurred again in exactly the same way. The only difference with the second deletion is the German version of post 3 has remained intact.

My web host suggested it may be an issue with the multilingual plugins. There were no problems before the German language work. The only changes in plugins, apart from updates, is an addition of the Classic Widgets plugin since WordPress 5.8 block widget editor did not work; could this cause incompatibility problems? I noticed my website theme Lodestar would not update https://3dresearchspecs.com/wp-content/uploads/2021/08/screenshot-36.png , even though it was telling me to update it to the new version, I am not sure if this matters.

Can you suggest any fix before I go in to uninstalling plugins and changing themes?

Additions to problem explanation for Polylang
?	Permalinks settings…W/P amin…Settings…Permalinks…Common Settings-Post Name
?	Not using a static front page
?	Polylang settings… URL modifications-The language is set from the directory name in pretty permalinks and -Remove language in pretty permalinks
Detect Browser Language is activated
Media is activated

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