You get an error 'the server block is not well formed' after changing or modifiying a DotNetNuke Skin.  This issue caused about a issue on the control registration in your skin or container control.  Mostly it is a misstype on a line with a space to much or missing or you have done any other misstype.

Check all lines on the top of your control and make sure there is no misstype.  If you have found the misstype and corrected it the error message will gone and your skin want work as aspected.