Would you both please locally extract your UPGRADE packages into a named folder and verify that you see the Telerik package "Taxonomy_05.06.00_Install.zip" in \DotNetNuke_Community_05.06.00_Upgrade\Install\Module.
Please verify that the "Product Version" is 2010.2.929.35"
This is what we shipped with 5.6.0 and what I get when I download an upgrade package from http://dotnetnuke.codeplex.com/releases/view/55790

If you got your Community Edition upgrade packages from some other location please advise

Thx,

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Ken Grierson
DotNetNuke Corporation
Test Lead

@kalle:
Do you happen to be using an Object Qualifier in your database?
Your .Net 4.0 error message is suspiciously close to XMLMerge errors we've logged before in Gemini.

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Ken Grierson
DotNetNuke Corporation
Test Lead

5.6 IS STILL BETA!  Please remove from stable!

I upgraded to 5.6 from 5.5 and I also did a fresh clean install. Both times 5.6 has been broken after installation.
Error Message:
A critical error has occurred.
Object reference not set to an instance of an object.

This is showing when I try to edit a portal from the host > Portals page.  When I view the website root there is the same error. On my upgraded site it caused the error to show up in every page and broke all my content. Every page where I had a content module the error was there and no content would show up. This is a huge problem for hosting providers like myself who are looking to DNN for an alternative for customers.