There's a random bug with the widgets : the ExternalOrResourceFilesWidget (web plugin) extends the ExternalFilesWidget from the cms plugin.
The ExternalOrResourceFilesWidget js file is referenced in the web plugin.xml without including the ExternalFilesWidget js first.
When the ExternalOrResourceFilesWidget js file is loaded first, it leads to an error when trying to edit a content.
The solution is to include the cms widgets in the web feature, before the web widgets extending it.