Some anticheat don't like DO's files. I don't have that problem with Apex (probably because I do not use DO hotkeys but with Fortnite it was giving me another type of error unless, as stated above by Leo, DO was completely shutdown (not only "closed" from view but fully "switched off").
I believe "my" solution was to tick a box
_Settings > Preferences > Launching Opus > Startup > Shutdown > Tick "Shutdown DO when the last Lister closes.
Might also have to disable a few things under:
Settings > Preferences > Launching Opus
- Explorer Replacement : Don't replace explorer
- From the Desktop : Disable
- From the taskbar icon : Disable
- From the Win+E hotkey : Disable
- Startup : Launch DO automatically on system startup.
All these settings, I believe, will keep a file (DLL?) actively "open/loaded" in system which anticheats wrongfully interprets as a "menace" or something.