disabling NextGen Gallery the error message disappears.
But I found that the post is still updated even when the message pops up. It seems a bit like a timeout because I found the message one time also with NGG disabled.
Yes, I know. For a while I could not update NextGen because it was not compatible with Rankmath.
Anyway, the actual problems using Gutenberg I had not with Elementor. Now I must clear the Autoptimize cache after every update because it does not take some Generateblocks formattings and I shall disable NGG. Does this mean that Autoptimize and NGG are not fully compatible with Gutenberg?
As Envira is not multilingual and I will not add new galleries, I will probably pass to the standard wordpress gallery for the existing galleries.