This can be caused by wrong file permission, file path or due to the security policy (via some plugin or hosting) blocking access to admin-ajax.php.
Also, you can try changing the default address of WordPress page http to https or reverse from General Settings.
If you want to re-install the plugin, deactivate and delete first and then install the new copy.
This process will not delete any of your data. Although to be safe, you can export UMP data first from “User Meta >> Export & Import >> Export UMP”.