[ad_1]
Deactivate all plugins for the time being. Does it work again then? I assume so. If it is so, activate the plugins individually again until the problem arises again. Then you should have found the guilty plugin.
If it is not a plugin, it can also be the theme. You can test this by changing to a standard theme.
Thank you for your answer. I have installed the classic editor again and now it is working.