Settings won't save

I have the plugin installed and can find the settings page under Settings>#hashtagger

When I change the @username and click the checkbox for @nickname instead of @username and then Save Settings, I get a message that the settings are saved BUT the @username field AND the checkbox have reverted back to their original status (no longer reflect my changes).

Not sure why or how I might help diagnose further but let me know if you require more information.

https://wordpress.org/plugins/hashtagger/