Constraints not accessible in storyboard on Xcode 12 beta 6

How can I access constraints in a storyboard in Xcode 12 interface builder?

One of my scenes (containing the cells of a UITableView) in a storyboard created back in Xcode 8 and maintained since does not show me the constraints attached to the views. They are still there, I can see and edit them in the outline view and the size inspector in Xcode 11.6, and I can see their effect in the running app (with a misalignment in iOS 14 that needs to be fixed).

I can also still see and access constraints in Xcode 12 in another scene in the same storyboard file, and in other storyboards and xib-files, older and younger. However, the constraints on the UITableViewCell instances are not visible, and thus not editable in Xcode 12.

Xcode occasionally issues the warning "An Internal error occurred" when I open this file. When I click on the "Report a Bug" button next to it, it just opens a Finder window pointing to some hidden folder within /private/var/ and hides the warning.
Can you open and see the constraints in XCode 11 ?

If not, may mean you have some file corruption. You should remove the views that cause problem and recreate them. Don't forget to remove the connections before if you do so.
Constraints not accessible in storyboard on Xcode 12 beta 6
 
 
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