I think you need to change all of them at the same time. EE submits all of them to the db, and if only one of them is invalid, that would trigger the error.
There’s no way, from Admin, to change all at once. There’s only access to one at a time.
Actually, it turns out the settings under Member Preferences > Avatar Preferences, Member Preferences > Member Photo Preferences, etc., can be modified all at once by changing each one and *not* clicking the update button, but instead clicking the next settings button on the left menu and updating the other paths first - and *then* clicking the update button. Since the menu settings are revealed via Javascript, you can switch through the menu settings (specific to Member Preferences) and they will retain your changes within the session / on the page until you click update to save to db.
I haven’t been able to replicate my previous environment that was causing problems to test this, but I have verified that settings under other menu sections within Member Preferences are retained w/o clicking the update button, so if indeed Ingmar’s hypothesis that all paths must be changed at once then this would be the way to do it.