An upgrade to AOL's instant messaging system has caused widespread disruption among its users.

The company started rolling out the upgrade to AIM - version 5.9 - on Thursday, a process which coincided with an unknown number of users in Europe suddenly going offline.

It is not clear what in the upgrade process caused the glitch, and it has not been a universal problem, but a significant proportion of users found they were unable to connect to AIM until the computer had been rebooted, despite the machine otherwise working without a hitch.

AIM users running other clients, including Trillian and Gaim, remained unaffected, raising the possibility that it was a notification from AOL about the upgrade that has caused the problem. AOL has so far failed to make any comment on the issue.

AIM Today 5.9 features a web browser that integrates with the AIM client, and offers a range of security features, including tabbing and anti-spyware and anti-phishing checks. Once downloaded, the upgrade process itself is believed to work smoothly.