An error occurred while collecting items to be installed
session context was:(profile=SDKProfile, phase=org.eclipse.equinox.internal.p2.engine.phases.Collect, operand=, action=).
Problems downloading artifact: org.eclipse.update.feature,org.strategoxt.imp,1.0.2.0-r24315.
MD5 hash is not as expected. Expected: d565a0d74fa5c80c5a43a5165a5cc74d and found 049ce81820ccb500fe2e9a16895d83e0.

Submitted by Eelco Visser on 29 June 2012 at 21:38

New installation causes a (similar?) problem:

Cannot complete the install because one or more required items could not be found.
Software being installed: WebDSL Editor 1.2.9.201206291720 (webdsl.editor.feature.feature.group 1.2.9.201206291720)
Missing requirement: WebDSL Editor 1.2.9.201206291720 (webdsl.editor.feature.feature.group 1.2.9.201206291720) requires ‘org.eclipse.jst.enterprise_core.feature.feature.group 3.1.1’ but it could not be found

Was the option ‘Contact all update sites during install to find required software’ checked? (in the new installation attempt)

That does the trick. (But then installation takes extremely long (hours).)