Share Logins Pro uses REST API to connect between sites. This error usually happens if you are using any security plugin that is not compatible with Share Logins Pro. Also, if the hosting provider has any security plugins installed for their host sites. Thus, you will get validation errors on Share Logins and the functionalities won’t be synced among sites anymore.
In these instances, you can try deactivating the WordPress security plugin and check if the sites are connecting or not. If they still don’t connect then you need to contact your hosting provider and ask if they have any site security measures for your sites by default.
If they do, you should ask them to deactivate any security plugins used. Then check the validation reports on both sites. If the error report persists try deactivating and reactivating the license key and then the plugins.