Plain-text files are usually displayed by our own viewer plugin, although it can be overridden.
If those are crashing, please check if any crash logs have been saved and send them to us: Automatic crash logs (for bug reports)
Also, it may be worth checking if the crash goes away if you turn off certain viewer plugins under Preferences / Viewer / Plugins. (The ActiveX + Preview + Office + Web plugin is the one most likely to be involved based on what you're described. That is really a meta-plugin which allows third party viewers to work in Opus. If you click Configure on it, you can see a list of the third-party viewers which are installed, and which file extensions they're handling. Or just disable it entirely to quickly test if any of them are involved.)