It's most strange that it happens when cancelling the dialog, since cancelling it essentially just closes the window without making any registry or configuration changes.
Do you have any tools installed which change the way windows behave or are arranged, or which modify normal mouse/keyboard behaviour or things like that?
Does the same crash happen if you double-click and then cancel things on other tabs of the filetype editor (e.g. Actions, Events, Drop Menu)?
My guess is that a tool or component is causing the problem, one which is loaded normally but not loaded when in safe-mode.
Since Opus 9 is also having the problem, and was working in the past, it has to be something else that has changed on the system since things were working.
Tracking down what it is may require some trial and error, though. I would try uninstalling anything you don't need/use anymore, in case it is related, and then try using ShellExView to disable any non-Microsoft shell extensions (then reboot; you can enable them again later if it doesn't help) in case that helps.
Yes, type /dopusdata into Opus to go to your configuration folder, then go into the FileTypes folder below there.
If there's already a file for the filetype in question then it may be fairly straightforward to edit it, but if not it could be quite difficult unless you're familiar with the way the filetype registry data is arranged.