placeBet() function of COM-interface

[Sergiy]

I am using COM-interface of Gruss Betting Assistant in my VB.NET program.
Everything is working fine, except of function "placeBet".
Call this function returns the empty string.
According to CommClass reference this function (placeBet) must return "bet reference or error message received from Betfair’s server".
I have checked all arguments for placeBet() function - everything as in CommClass reference.
But it returns just empty string.
No bet reference, no error message - just empty string.

Please advise, what could be wrong in call placeBet() that cause to return empty string?
Thanks in advance.