since WordPress latest upgrade, none of my right sidebar widgets are accessible, either through the “Appearance => Widgets” menu or through the “customize” interface, where they appear but cannot be changed.
you were right, I found that the GUTENBERG plugin was creating the problem. I deactivated it and things were back to normal.
I noticed that if I reactivated it again then things would also be ok, although the screen for Widgets would be quite different. Do you think it is necessary to keep that Gutenberg plugin ?
Glad to hear you found the issue.
No you do not need the Gutenberg plugin – it is a continuous Beta plugin that allows uses to test out new features that will be inserted into the core editor at a later date. With that comes bugs like this, which will be resolved before thy are pushed into core.