I have a some ISO file that can be mounted by Daemon Tools, can be opened by PowerISO and UltraISO, but cannot be opened by DOpus, WinRAR, 7-zip.
After mounting it does not look unusual - plain ISO9660/Joliet.
If DOpus Team is willing to investigate this issue I can upload this ISO to FTP and provide link.
ISO handling is done via 7z.dll so if 7-Zip itself cannot open the file then Opus won't be able to either. (Except when you mount it via Daemon Tools etc., of course.)
We'd only be able to fix ISOs that 7-Zip could open but Opus could not for some reason.