The Independent Parliamentary Standards Authority (Ipsa) has moved to calm anger among MPs, promising more face-to-face meetings.
But in Parliament yesterday, MPs complained the organisation was too bureaucratic and had resulted in serious breaches of privacy.
Labour's David Winnick (Walsall N), a leading campaigner for greater transparency in the expenses system, said it was "very difficult indeed to get any sense out of Ipsa".
Opening an unusually well-attended debate in Westminster Hall, he said: "It is indefensible that Ipsa should have set up a system which is so difficult and so complex, and, particularly for new members, has made life a nightmare."
Mr Winnick, who repeatedly hammered on the table in anger as he made his speech also hit out at the number of spin doctors being recruited by Ipsa, including a communications director earning up to 85,000.
"You do ask yourself why on earth Ipsa should require three spin doctors, why any spin doctors?"
Phil Wilson (Lab, Sedgefield) was given an ironic round of applause by MPs as he said he had succeeded in logging on to the online expenses system.
But, he added: "The expenses that appeared on the screen were those of another MP."
Shadow Justice Secretary Jack Straw, who steered the legislation setting up Ipsa through the Commons, was heckled by MPs as he tried to justify the body.
He said Ipsa had "failed to take account of the reality" of the way MPs work and the need to set up their own offices.