Please try disabling the Audio Tags plugin under Preferences / Viewer / Viewer Plugins.
If that stops the problem, please zip and send one of the FLAC files (after checking that that file on its own is enough to trigger the problem when the plugin re-enabled) for me to look at. If there isn't a small one then it may be too large for the forum, but you could email it to leo@gpsoft.com.au if possible/convenient.
If disabling the plugin doesn't make a difference, the problem is probably in a component on your system that isn't part of Opus itself. Crash, exit or high CPU when viewing certain directories has further suggestions if that appears to be the case.