Plesk with billing is insalled.
Login to admin panel:
https://[pleskurl]:8443/sso/ui results in the following error:
Error 'Invalid signature'
https://[plesk_url]:8443 redirects to special sso interface
https://[plesk_url]:8443/relay with SAML request, and then, it redirects to
https://[plesk_url]:8443/sso/ui, and sets variable signature.
This is Plesk internal issue PPPM-1223, which is expected to be fixed in future product updates.
https://[plesk_url]:8443 to log in to Plesk.