The problems appear to be connected to an SSL security certificate used by Omniture Inc., a company PayPal is using to collect aggregate data about people using its Web site, the PayPal spokesman said. SSL certificates are used by Web browsers, such as Internet Explorer and Firefox, to check the authenticity of a site and to let users know whether a site is spoofed or not. The spokesman said the problem affects a "small portion" of PayPal users.

Omniture started using a new so-called wildcard SSL certificate on Jan. 29 that is causing problems for users of Windows 98 systems and even some newer versions of Windows when they attempt to access PayPal. "Users running outdated operating systems, like Windows 98, as well as some that are using more recent software are seeing these warnings when they use the PayPal service," the spokesman said. The company is working with Omniture to diagnose the problem and to fix it, he said.

Using Firefox to access PayPal provides an immediate work-around for all users, he added.

A wildcard SSL certificate allows a company to use a single certificate to authenticate multiple subdomains within a larger domain.

Several users on a PayPal forum on eBay have complained over the last few days about error messages popping up on their screens warning about the validity of the certificate issued to the site.