I noticed strange behavior in backend. Every created layout shows blank page in sandbox. After I add some second layout of the same table, it works, it means only newly added. So then I remove the previous not working one.
But when I add new table and layout everything is gone (blank page again) including the working layout of the previous described step.
Many hours I spent on this to figure out what I am doing wrong.

By the way, the interface is realy demanding on comp memory during work and testing in sandbox. So it needs to free memory from time to time

etc

I figured out one thing about blank page in backend. When I have filter by username it always fails.

Note: sandbox 3.0

edit:

I forgot to mention, the "username" in filter is always automaticaly replaced to "last visit date" after refresh builder.

Try again please.