There should be no need to d&d it, if you already have the file in your "usermacros" directory. If you have that directory configured in "Customize -> Configure User and System paths -> Additional Macros", it will be evaluated at 3ds max startup. So just delete the old button, close max (by default, UI is saved when closing max), start it up and go to "Customize User Interface" -> "Toolbars" to add the icon to some toolbar.
Thanks for your help Frood.
I confirm the usermacros directory is correctly configured in "Customize -> Configure User and System paths -> Additional Macros" and RTstudio-RenderElementsOrganizer.mcr is here.
I deleted the old button, deleted the toolbar ("Customize > Customize User Interface > Toolbars" tab), I restarted 3ds Max and the old button was here again with the same message.
When I go to "Customize > Customize User Interface > Toolbars" and I search in "Category" I can't see RTStudio anymore.
So the issue looks like the .mcr is not evaluated at the start of 3ds Max for some reason, although it is present in the usermacros directory.
I don't know what to do.
Of course, at that point I can drag and drop the mcr on the viewport and now RTStudio appears in the category and I can drag the script to the toolbar and the button works, all good. But as soon as I restart 3ds Max, RTStudio category is missing again.
To be on the safe side, I even saved the default state of the workspace before restarting Max, just in case, but the problem remains at the next restart.