Thanks for adding this to the thread.
I did eventually find the cause of the issue, but of course not until I'd restored and redone a lot of content. It's still sufficiently obscure that I don't know exactly why this happens.. it just doesn't make any sense to me.. but here it is:
The background: Ever since the new user profiles in DNN4, there have also been new 'required field' images in place of asterisks. These are ugly round red arrows that display very badly on non-white backgrounds. In addition, on certain skins, those images slip underneath the text fields and upset the layout of the page.. not to mention they then no longer point at anything.
So I have always done a couple of small things to my DNN instances. First I replace the arrows with my own tidier graphic... and second, I make a small change in User.ascx
I wanted to make it so the images don't slip underneath the fields, but because the dnn:propertyeditorcontrol has no table cells where I could simply set nowrap, I discovered I could stretch it out a little sideways with the editcontrolwidth and width settings already present. I would simple add a few pixels to the values.
Now that might seem pretty tame, and it worked perfectly for a long time, but then it caused the namespace error that started this thread. The error would occur only with certain skins. I tried all sorts of different values and it seemed to fail when the skin squeezed the control too tightly or something.
It's a mystery to me why it stuffs up and why it gives that namespace error message, but it absolutely does. I did it over and over again... simply changing the values a few pixels would make it fail. I confirmed it on a seperate DNN instance as well. So now I'm not touching it.
I did of course restore the core user.ascx file at least 2-3 times when I was posting earlier, but it didn't happen so clearly then and there must also have been some caching that was causing me to see the modded file... and as well, I would never have suspected this to be the cause. When I finally discovered the issue, I had to manually clear the cache each time to check for the error.
So.. if anyone does knows how to reliably widen the control, please let me know :)
And thanks to everyone who tried to help out.
Rob