We’ve had a report about this previously, and the issue was caused by the web browser. Either a browser extension/plugin or some configuration setting.
Can you please try a different browser and in the Incognito/Private window so that there are no browser extensions running?
If that doesn’t help, please try temporarily deactivating all plugins and extensions in WordPress except for MainWP Dashboard and Wordfence extension, and see if that helps.
And just to be sure, the extension version you have installed is 4.0.6, and the MainWP is 4.2.6?