Solution found - I had to use the "create user" command with the "using" clause:
SQL> create user SYSDBA password 'masterkey' using plugin Srp;

My firebird.cong indicated that Legacy_Auth is the first (default) user manager and it was necessary for isql to know that Srp manager had to be used:
UserManager = Legacy_Auth, Srp

So - my conclusions about the improvement of the Firebird Release notes:
p.119 of the PDF document contains important notice:
"This initialization is not required and should NOT be performed if you have configured the server to use legacy
(pre-Firebird 3 style) authentication and user management."
Actually this is not true! Clients want to check 'Enable authorization for legacy Firebird clients' and they want to use Firebird 3 gds32.dll/fbclient.dll and in such cases the initialization is required.

p.119 also provides the example of create user command. It could be helpful to add note about "using" clause. Without such notice the initialization procedure can not be completed as described.