"Error on Layout Not Found".
For further information.
This problem is known and has already been encountered in 2020 by other users.
See the support response here: Error on layout not found.
It is 8/2/2021 and I have just had this anomaly with our CMS in V2.3.8.
The Layout cannot be edited, but the preview is correct and it is possible to create a copy of the layout in error.
The solution will be to replace the layout in error with its copy. When the copy is validated, the erroneous layout must be deleted.
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