This is becoming serious, because datbase has several hundreds procedures and some of them have the same situation (date arguments and variables) and have the same problem. Problem manifsts only when free statement is issues. Problem disappers when the procedure is changes from the IBExpert procedure edit form. But still the following situation happens (on clean server - that is restarted):
- I execute mentioned statement in IBExpert editor, IBExpert immediatly gives error message:
    "Unsuccessful execution caused by a system error that precludes
     successful execution of subsequent statements.
     Error reading data from the connection.
     Connection will be closed immediately."
  The connection becomes cloased.
- I reopen connection and go to the procedure edit form and execute edit from this editor - the error message appears:
    This operation is not defined for system tables.
    unsuccessful metadata update.
    unknown ISC error 336397267.
    deadlock.
    update conflicts with concurrent update.
    concurrent transaction number is 720. 
Only when I restart server I can edit procedure and commit changes of procedure from the procedure edit form.

There seems to be some problems with transactions...