I've had similar problems which were all solved when I deactivated User Account Protection which can be done by any admin in the user account settings and just needs a restart. It somehow blocks almost all overwriting of files without notification. If you don't want that, you can change the user access for each and every file/folder(may work by setting it for all subsequent files and folders, but is still hassle), but I'd personally just deactivate user account protection which should fix your issue.