would a plugin cause this problem?

To find a plugin or a theme conflict, please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable them one by one to identify the source of your troubles.

If you can install plugins, install “Health Check”: https://www.ads-software.com/plugins/health-check/ On the troubleshooting tab, you can click the button to disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site.