Error when updating eclipse

I had the same problem with Eclipse Helios (3.6) on Ubuntu 64 bits.

Removing

http://download.eclipse.org/tools/cdt/releases/helios

from the "Available Software Sites" in Eclipse, and adding it again did the fix.

Though this question is answered a long back, wanted to share my experience thinking that it may save some one' day :)

Some times it could also be possible due to the incorrect proxy settings. In my case (Eclipse 3.6/Helios), Eclipse preference file $ECLIPSE_HOME$/configuration/.settings/org.eclipse.core.net.prefs is having proxy details,

proxyData/HTTP/hasAuth=true
systemProxiesEnabled=true
proxyData/HTTP/host=proxy.domain.net
org.eclipse.core.net.hasMigrated=true
eclipse.preferences.version=1
proxiesEnabled=true
proxyData/HTTP/port=9119
nonProxiedHosts=

When I deleted the file after closing the Eclipse, then onwards I m able to download the plugins

What version of eclipse are you using ?

It has been fixed in 3.4.1, but in your case, it could be this bug instead.

It turns out the cause is one or more artifact repositories being lost from the list of known repositories.

The workaround is to remove the repository from the list and add it back again. This restores the missing artifact repository.

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A possible workaround in eclipse 3.4.1:

1. export the update site listing to bookmarks.xml file (Software Updates->Manage sites->Export)
2. stop eclipse
3. remove configuration/.settings/org.eclipse.equinox.p2.*.prefs files
   (Note, as mentioned by Fredrik in the comments, for Eclipse3.5+, this would be:
   p2\org.eclipse.equinox.p2.engine\.settings
p2\org.eclipse.equinox.p2.engine\profileRegistry\SDKProfile.profile\.data\.setti‌​ngs)
4. start eclipse
5. import the bookmarks.xml file (Software Updates->Manage sites->Import) that was exported in step 1

As mentioned by mark (author of this question) in the comments of this answer, that workaround was a solution for his problem.

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Other advice:

[...] the solution is always to append a trailing '/' to all repository URLs in my P2 UI.
My feeling is that this error is caused by a failure to construct a viable URL to a repo resource. The code in question is not inspecting the URL string from the UI and is assuming a trailing '/' resulting in a 404

This last solution is confirmed working in Eclipse Indigo and Juno (below in the comments).

Yet another solution -

Instead exporting all managed sites and importing them back, click on 'edit' on each checked site and add '//' at the end

If you just add a '/' it says duplicate site therefore the '//'

How a small bug in a huge carefully architected application can be so hard to debug yet simple to solve