Hello!
I'm hoping someone out there might help me keep some hair on the top of my head as I've been slowly ripping it out trying to figure out what is going on.
I'm having the common "Controls collection cannot be modified" error -- but not in a common way (at least I don't think it is)...
At first it was because I had misspelled ContentPane -- however I found that one and am now left with the following (listed by steps):

I install DNN all brand-new like.
I upload my skin which has 1 default and 1 admin page (currently in HTML form -- not ascx if that matters).
It gets parsed just fine and I apply the default and admin skin.  So far so good.  Everything is going great.

I log out, close my browser, and re-open it all.  The default page comes up fine.  I try logging in  as host and get the Controls Collection error.  Now I've gone through the ascx file that got created for admin -- as well as the default one and both look just fine.  No img tags inside a runat=server, etc.  But there it is -- that silly crazy error.

The only difference between the default and admin skin is that I got rid of a couple extra content related panes (LeftContentPane and RightContentPane).  Please!  Is there something obvious I'm missing?

In advance, my hair thanks you,
Brent