The software maker said some users of ActiveSync 4, the latest version of Microsoft's synchronisation software, are unable to link up to their PC, particularly on systems running firewall or parental control software.
The problem occurs in devices based on Microsoft's Windows Mobile 5 operating system, including recently released handhelds from Dell and HP, as well as mobile phones from Sprint and iMate.
The problems are limited to those connecting a device to a PC via a USB connection, Microsoft said. No problems have been reported when connecting wirelessly using a Bluetooth or infrared connection, or when connecting directly to a server.
"Microsoft is committed to solving this issue for its customers and its partners," the company said in a statement.
An updated version of ActiveSync is planned for late next month, but a workaround has been posted on Microsoft's mobile device Web site, the company said.
