1. Web Site Migration is started at Plesk > Domains > example.com > Website Importing.
2. Progress bar of establishing connection has been minimized by a user.
3. "Please wait" button becomes "Ok". It is possible to submit FTP credentials once again before the migration finished.
This issue has been considered as Plesk bug #PMT-3882 and will be fixed in future product updates.
Ignore "Ok" button since the connection is already established.