Probably only someone from DO crew can help you, but still I can try to guessing - if your account is not admin account but just user account, then even UAC can't give you access to directories that have permissions set to admins only. UAC option in Windows gives you extra protection (or, to be more specific - adds extra annoying questions) but not rights to access admin only directories. I think run DOpus as another user ("runas" command) which has administrator priviledges, gives you possibility to copy files into admin only folders.
Does the window titlebar at the top say "ADMINISTRATOR" in all caps next to the folder name?
If you do the same copy using File Explorer, does that trigger a UAC prompt, or does it have the same result?
It triggers an UAC prompt.
Does doing anything trigger a UAC prompt, e.g. launching RegEdit.exe?
Are you definitely using Opus Pro if you check via Help > About Directory Opus? (The Admin Mode button would only appear in Pro, although general UAC support should work in both Pro and Light, e.g. when copying to folders that trigger a UAC prompt that should work in both versions.)
Check that compatibility mode hasn't been turned on for dopus.exe. e.g. If Windows is telling Opus that it is running on Windows XP then all the UAC support would be disabled.
Re-running the installer over the top of the existing installation, then rebooting, is also worth a try, in case the problem is one of the DLLs involved in UAC support has had its registry settings damaged.