Installation of Helios' XML Editor into Helios based Eclipse fails

Question

I am working with IBM Rational Function Tester 8.3, which is Eclipse 3.6 (Helios) based. I wanted to install XML support, so I enabled the Helios update site and chose Eclipse XML Editors and Tools 3.2.3 to install. I was promted for license agreement and then the installation started as expected. After some time downloading things Eclipse showed me an error.

An error occurred while collecting items to be installed
session context was:(profile=bootProfile, phase=org.eclipse.equinox.internal.p2.engine.phases.Collect, operand=, action=).
No repository found containing: osgi.bundle,org.eclipse.core.resources,3.6.2.R36x_v20110628-1020

I also tried Eclipse XML Editors and Tools 3.2.2 with the same result. I verified that org.eclipse.core.resources_3.6.2.R36x_v20110628-1020 is in the plugins folder of the installation and that it is loaded in the plugin registry. The needed plugin is available and active.

I have no idea why this happens. Please someone can shed some light into this issue. What could cause this problem? Thank you.