Error message when wanting to change the order status

I couldn't find any Thread that handles this problem, so I hope someone can help me.

I wanted to insert new order statuses. So I created the new ones and the old ones (that have been there from the start) seemed to not fit in my scheme anymore, so I deleted them. Then I changed the order of the statuses. Everything worked out well.
But now, when I'm testing to buy something, I cannot change the order status anymore. By changing the status the following message appears: "WARNING: An Error occurred, please refresh the page and try again."

Can anyone tell me why this error occures and how I get rid of it?

Thanks in advance!

Searching (via google) for zen cart "WARNING: An Error occurred, please refresh the page and try again." yielded this FAQ that will help: http://www.zen-cart.com/content.php?311-warning-an-error-occurred-please-refresh-the-page-and-try-again

Thanks for that hint.

If anyone has the same problem - this is how I solved it:
It turned out that I installed any COWOA codes in my order.php (can't imagine how they got there). I found it via Developer's Tookit. Then I replaced the order.php with the original order.php - and now everythings working perfectly