DavidT:
"Not sure what 'unblock' the DNN install package means."

The Windows Server 2003 Service Pack and all Windows 2008 Servers now have a neat little security mechanism whereby they block files by default.
You get the files, they just cannot be used until you "unblock" them.
If you right click the DNN installation zip file you downloaded and select Properties you might see an "Unblock" button bottom right corner above the "Apply" button.
if you have this button then all your files within this zip are blocked and if you unzip and use them before you click "Unblock" on the zip Properties windows all those files you unzip will be in a blocked state.
You can successfully install with these files but... anything that requires .NET Framework access will cause your site to fail.

That said I have no doubt there are lingering issues with running on .NET v4.0 as the first test pass to verify DNN ran with ASP.NET v4.0 installed and active was back in early spring of 2010.
W2K3, IIS 6, SQL Server 2008 and ASP.NET v4.0 was a scenario we tested however they were clean install and upgrade scenarios we didn't test with a large number of 3rd party modules installed.

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Ken Grierson
DotNetNuke Corporation
Test Lead

I too am getting this error on three sites out of nine converted from 5.05 to 5.06.

_________________________________________________________________________
DotNetNuke Error

Windows Server 2003

Button [Return to Site]
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On two of my sites when I click on the button the Home page displays and all seems fine

On the third site when I click on the button I get:

The Home page displays with the following:
A critical error has occurred.
The string parameter 'tagPrefix' cannot be null or empty. Parameter name: tagPrefix

If I then select a another page and then back to Home all appears correct

I've got the same problem. I thought it is something wrong with my web server or my installation. I installed the nuke again but the problem remains.
win server 2003 iis 6 framework 2
it is critical. what should I do?