ok, the problem is that you cleared the textbox which defines the whitespace compression filter. What you should have done is just clear the checkbox labelled use Whitespace filter in the Performance Settings section of the page to disable it. You're getting the error because the code expects that text box to have some value even if WhiteSpace compression is not being used.

The compression settings are stored in a file named compression.config stored in the root folder of your DotNetNuke web site, and should look like this, use notepad to open compression.config and replace the contents with the following...

http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/54/threadid/97183/scope/posts/threadpage/3/Default.aspx

That should do it. Go back into Host->Settings, open the Performance Settings section and clear the checkbox for use Whitespace filter. Click on Update, then Click on Restart Application.