I am trying to edit a Custom Field Group. But I can't do anything: I can't see field types, or see what fields are in the group. See screenshot https://snipboard.io/uvPpSl.jpg
Please try to switch to the default theme of WordPress (Twenty Twenty), deactivate all plugins except Meta Box and MB extensions, then activate plugins one by one. This procedure will help us to find the plugin/theme conflict with Meta Box.
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