Looks like you should ask the fusion core plugin folks. There may be an update. I recommend asking at https://www.ads-software.com/support/plugin/fusion/#new-post so the plugin’s developers and support community can help you with this.
Did the plugin fix your issue? If not, you can remove it and try the other troubleshooting test.
This may be a plugin or 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 JUST you, while you’re still logged in, without affecting normal visitors to your site. You can then use its admin bar menu to turn on/off plugins and themes one at a time. See https://make.www.ads-software.com/support/handbook/appendix/troubleshooting-using-the-health-check/ or https://rankmath.com/kb/check-plugin-conflicts/ for more information.
See https://www.wpbeginner.com/plugins/how-to-deactivate-all-plugins-when-not-able-to-access-wp-admin/
