invalid transaction handle (expecting explicit transaction start)

Hello,
i have a multi-tiers application using :
    - SOAP
    - Delphi6
    - Interfase (firebird 1.0.)
    - dbExpress component.

first page of my web server includes two frame and each frame call the
remote data server using soap.

And when they try to connect, this error appears :
    invalid transaction handle (expecting explicit transaction start)

if i put a sleep(1000)  in one of them error doesn't occur.

And it seems that more computer is slow, more error occured.

thanks for your answer.

ROSSI Jean-Luc