Hello Carlos, I wanted to ask if you are still experiencing this issue? I have been working with a user that is dealing with the same exact thing. After a clean uninstall/reinstall of ArcMap, we were able to resolve the "Add Data" folder issue. Everything seemed to be back to normal, but now the error message has resurfaced. From the screenshot you posted above, it appears there might some 3rd party extensions on your toolbar. This could be your problem, although my user did not have any installed.
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