Connection Errors

This looks to be a great plugin – especially as it includes the option to sync only users with specific roles.
Unfortunately, I can’t get it to connect! I’ve tried on multiple sites but get a selection of error messages – always the same error message when trying to connect a site but different depending on the 2 sites I am connecting.

1st 2-site combination setup:
Connection Test Failed: cURL error 28: Operation timed out after 20001 milliseconds with 0 bytes received – when using the admin Password
Connection Test Failed: Authentication with provided Application Password failed. Server responded with status code 500. – when using the application Password

2nd 2-site combination setup (different sites):
Connection Test Failed: Failed to generate Application Password. Remote server returned HTTP status 403. Please check admin username/password and ensure Application Passwords are enabled on the target site. – when connecting to the ‘parent’ site with admin password.

Connection Test Failed: Authentication with provided Application Password failed. Server responded with status code 403. Please check the username and Application Password. Ensure the Application Password is not revoked on the target site. – when connecting to the ‘parent’ site with application password.

Connection Test Failed: cURL error 28: Connection timed out after 10000 milliseconds – after adding whitelist authentication to the target site.

Usernames and passwords have been double checked and application passwords are enabled. Application passwords were created and tried with/without the spacing between the 4 letter combinations.

Any suggestions?