Jon Danzig, who started volunteering for the charity after being diagnosed with a pituitary disorder, complained to the commission last year that staff and trustees of the health charity had sent derogatory emails about him to members.
According to the report, Danzig said documents released to him by the charity after an intervention from the Information Commissioner’s Office showed the organisation had described him to supporters as a "master of misleading information".
The independent report on Danzig’s complaint has been published by the Centre for Effective Dispute Resolution, after Danzig asked the body to investigate the way in which the commission handled his original complaint about the charity.
It says the commission should have asked Danzig for supporting evidence when he made the complaint, and should have given him a clearer explanation of why it decided not to look into his allegations.
It also criticises the regulator for writing to the charity to say that, contrary to a previous letter, it did not "require" the charity to mediate with Danzig but did "recommend" that it did so. Danzig had complained that the change, which he was not aware of, undermined his attempts to enter into mediation with the charity.
The new report, written by the complaints reviewer Graham Massie, says: "Clearly there were serious deficiencies in the standard of service provided to Mr Danzig. The errors in the correspondence were unacceptable and not informing Mr Danzig of the amended letter was a serious failure."
In the report, Massie also says it was "unnecessarily unhelpful" of the commission, which offered Danzig a £100 consolatory payment earlier this year, to refuse his request to give the money straight to a charity. Danzig said that because he was receiving benefits it would have been an administrative burden on him to account for the payment before giving it away.
A letter to Danzig from the regulator’s chief executive, Sam Younger, says the commission has "reconsidered this point" and is prepared to donate the £100 to a charity. In the letter, Younger also apologises for transferring Danzig’s case from Jodi Berg, the regulator’s former complaints reviewer, to the CEDR, which in March won the contract to carry out the function. Danzig said the move was a serious inconvenience.
In a statement, a spokeswoman for the commission said: "We recognise that, given the nature of the concerns raised, it would have been better to ask for any evidence at the outset. However, having since viewed the further evidence submitted by Mr Danzig, we do not believe that it would have changed our decision not to intervene.
"We have acknowledged that errors were made in correspondence, and have apologised for them."
Danzig told Third Sector he was dissatisfied with the report. "A small forest has been lost in the pursuit of my original complaint, with hundreds of pages regurgitating my concerns, but to what effect?" he said. "I’m unsure if anything worthwhile has been achieved."
Danzig said he was considering whether to ask his MP to pursue his complaint with the Parliamentary Ombudsman.