There's no need to reboot.
Right-click the Opus tray icon and select Exit to shut it down. It'll restart again automatically when you double-click a folder or the desktop.
Opus 9 also lets you configure it to exit automatically whenever no lister windows are open.
It may not be Opus itself that was holding the locks. All of the shell extensions (including WinRAR's context menu, and all the others) in the system get loaded into dopus.exe, so if one of them creates a lock it will be attributed to dopus.exe. Same with viewer plugins, video codecs, etc. etc. That isn't to say it definitely isn't Opus either, but it could be any number of things.
If you can work out a reproducible sequence of events that results in a lock like that then you can use a process of elimination to work out what is causing it. This FAQ describes how to disable individual context menu handlers, for example. Disabling viewer plugins can be done easily through Preferences->Plugins, too.
If you do uncover anything let us know.