We are not sure what causes that but there seems to be something outside of Opus which is editing Opus's config files to make them use ACDSee. Some people have reported it even on machines they swear have never had ACDSee installed. It's quite strange.
Either that or people are using a shared config from somewhere/someone which includes that setting, but we haven't been able to find any config on the forum or elsewhere which does it. If you have any ideas, let us know, as it'd be great to get to the bottom of what is doing it.
If you use the current beta version of Opus, it should automatically detect this now and will ignore that part of the configuration (if ACDSee32.exe isn't actually installed). Just installing the beta should be enough to solve the problem.
Alternatively, you can fix it by editing the file type: