The problems stem from a feature of the online marketplace’s computer system that is designed to send each insurer a daily report listing people who have just enrolled. According to several insurance industry officials, the reports are sometimes confusing and duplicative. In some cases, they show — correctly or not — that the same person enrolled and canceled several times on a single day…

For instance, one major insurance carrier, Cigna, sent a notice Wednesday to insurance brokers instructing them to wait until November to try to sign up customers who might qualify for a subsidy, according to Joseph Mondy, a Cigna spokesman. He said that the company does not yet trust the reliability of the part of the exchange that is supposed to calculate the tax credits that will, for the first time, help some Americans pay for private health coverage…

Compounding the confusion, these electronic enrollment files are missing a critical element that they were built to include: a time stamp that would let a health plan track whether a consumer’s last step on the site was to actually sign up.

Insurers are uncertain of the cause of the flaws, with some speculating that the exchange cannot consolidate consumers’ moves through the Web site as they shop for and ultimately choose a health plan. Others say the problem could be unrelated software errors.