As reported here [1], you are not able to edit a previous version of a beta.wmflabs.org talk page.
Expected behavior
- Go to a prior version of the talk page you are viewing (e.g. https://en.wikipedia.beta.wmflabs.org/w/index.php?title=Talk:Cats&oldid=407990)
- Click "Edit source"
- Make changes
- Click "Publish changes..."
- ✅ Changes successfully published to page
Actual behavior
- Go to a prior version of the talk page you are viewing (e.g. https://en.wikipedia.beta.wmflabs.org/w/index.php?title=Talk:Cats&oldid=407990)
- Click "Edit source"
- ⚠️Unable to make any changes to the page