Exception when upgrading eclipse plugin

I got an exception when i upgrade the plugin into eclipse

An error occurred while collecting items to be installed
session context was:(profile=epp.package.jee, phase=org.eclipse.equinox.internal.p2.engine.phases.Collect, operand=, action=).
Comparison method violates its general contract!
Comparison method violates its general contract!

Which Eclipse version are you using?
Did you choose “Help → Check for Updates” and update all relevant plugins, or use “Install New Software…” and possibly only select some?

Eclipse Java EE IDE for Web Developers.

Version: Helios Service Release 2
Build id: 20110218-0911

I do both install new software and check for updates with same result.

This might also be related to running Eclipse with JDK 7 - see the comments in
Eclipse bug 37785
- but I see you are already following the status of that ticket.

I am using the same eclipse version, but JDK 6.

Version: Helios Release
Build id: 20100617-1415

and I get a similar error, when trying to install vaadin eclipse plugin. What should I do?

PS: The stack trace in eclipse :

This is a different problem, and as a little Google search shows, a few people have been having this problem when installing various Eclipse plugins unrelated to Vaadin - AJDT, Spring IDE, Google GWT plugin etc.

My guesses:

Most likely, you don’t have some required (standard Eclipse) update sites selected in Preferences → Install/Update → Available Software Sites (make sure you have at least the “Helios” and “The Eclipse Project Updates” sites selected; having “The Eclipse Web Tools Platform (WTP)” selected is probably also a good idea, and of course the Vaadin site should be selected). If you have installed some Eclipse bundle other than the Java EE bundle, you might have to add the WTP site (http://download.eclipse.org/webtools/repository/helios) to the list by hand if it is not there - I’m not sure about this.

Alternatively, this could also be a network problem (maybe with some proxy or firewall), or perhaps some invalid data in your Eclipse update system cache.