The latest version of WordPress supported by MemberMouse is:
NOTE about WP 5.3.x - WordPress 5.3.x was released on November 12th. We have identified an issue with the new admin email verification feature in WordPress 5.3.x and any version of the MemberMouse plugin. When an admin logs in using the MemberMouse login page, they are redirected to the Error Core Page, and will see "on=confirm_admin_email" appended to the end of the URL. They can proceed to the WordPress Admin dashboard manually, by going to yoursite.com/wp-admin. We are working on a fix for this issue to be included in a future version. In the meantime, to prevent this from happening in future, you can temporarily set the WordPress login page from MemberMouse > General Settings > Other Settings >, login again to verify your admin email address, and then reset the MemberMouse login page as default. If you have any questions, please feel free to contact our support team.
NOTE - Before upgrading to 5.0, please view our article on What changes can I expect if I upgrade to WordPress v.5.0?
It is always best to have a current site backup before upgrading your version of WordPress. Your hosting provider should be able to assist you with securing a backup.
Downgrading to a Previous Version of WordPress
WordPress Release Archive