This is not GYB fault, but you may be the only one who can help.
I made several attempts with Google Takeout to download a backup of groups messages (posts) as mbox file. I was only able to obtain the mbox of one pending message, no other file is added and a red message appears in the "Manage your exports" page:
"We weren't able to fetch all the data you requested. The list below highlights the services that had problems. You can still download your files, but please be aware that there will be data missing. Inside your export you will find an html file that will have more details about the errors."
In the html file I do not get much help:
"Scanning error– some of your items were not found and could not be included in the archive.
Some file formats (e.g. html, png) will be viewable in your browser, and will appear as clickable links below that will open the file once you've fully extracted your archive. Other file types can be opened by their appropriate applications by navigating through the archive's directory. You can click on any folders that may be present to see their contents.
Data that are still in the process of being deleted are not included in your archive.
Learn more about your deleted data."
I also had a try at the data export function of Google Workspace Admin Console with similar results, errors about not being able to read all elements of groups.
Of course I used a super-admin account that is also owner of the group.
Opened a question here, but no answer: