Hi David,
Thanks for your quick answer. The second test was the first thing I’ve tried, after refresh nothing happens and editor width keeps that narrow layout.
And regarding disabling plugins, this is a production site, therefore I only have GP, GenerateBlocks, and of course Gutenberg. Anyway, after trying to disable GP and GenerateBlocks, the problem persists. I think it is due to the last Gutenberg update, some weeks ago was working so nice, full-width editor and plenty of space to design…