After applying the 8.2.0 update, certain environments may experience impeded function of the Login to Panel button on Plesk services.

Please apply the 8.2.1 Maintenance Release via the Utilities > Update WHMCS page.

Enable Allow IP address changes during a single session under  Plesk > Tools & Settings > Active Plesk Sessions > Session Settings on the Plesk server.                                  

A hotfix has been created for 8.2.0 only to correct this behaviour:

https://whmcs.community/files/file/168-core-16812-plesk-login-to-panel-sso-ip/