All plugins & custom scripts are inactive
Occures in version 6.3.2, but also did in 6.3.1
There are contents hidden in the gridder, but still visible on the front-end.
Example:
I created a site with a thumbnail grid. When doing so, everything works fine. I can place and edit the grid as usual.
Note: The "null" element on the screenshot is an example.

Right after saving and re-opening:
The gridder is empty, or at least it hides the content of the grid.

There is still a chance to edit the thumbnail grid, by clicking on the anchor points of the thumbnail grid (or what is left of it)
Doing so, even the edit-menu does not show any content any more.
Note: I am trying to make a structure that is identical between WooCommerce and projects. The sites and grids are projects, not product thumbnails.