Thank you Ray, that did the trick. It's all working again. One odd thing: even when I renamed the waue_donation_1.php file, there were still instances of reference to that file name deep in the code. I didn't know if I should rename all of those too. I didn't, just to see if it would work. Luckily it did without any further intervention.
I realized the thank_you.htm file wasn't being found because it needed the ../ in front of it. Duh!
Once again, you saved me!