When I'm in Windows 10 File Explorer and right click on a folder, Bitdefender shows in the context menu and I can select it to scan the folder for viruses. When I do this in Directory Opus, Bitdefender is nowhere to be found. What is causing this error and how can I fix it?
BitDefender have chosen to hide their context menu from everything except Explorer.exe.
If BitDefender has a command line or scripting interface for scanning files then you could add your own item to the context menu which uses it and would work in Opus. Failing that, you will probably need to take the issue up with BitDenfender.