This shouldn't have anything to do with the button code in your page, so there's no need to concern yourself with that. The button code in the test page you linked looks fine. However, we were unable to reproduce any error from your test page.
Just to confirm, is "Invalid Processor" the exact, entire error message you're seeing? If not, please copy the complete error message you get and paste it into a reply here. I'm going to ask Development to look into this, since the last time an "Invalid Processor" error started appearing, it did turn out to be a glitch we needed to fix on our end.
The & thing is actually fine, as well. That happens because the "&" character has a special significance in HTML, used to indicate when you're inserting a symbol (e.g. &Â© inserts a copyright symbol). Technically if you have an "&" in the raw HTML code that should be treated as a literal "&", you're supposed to use "&&" (meaning "insert an ampersand symbol here), so many page-editor programs will do that automatically, but all major web browsers are sophisticated and lenient enough to recognize the difference anyway. We leave it as a plain "&" in our button-code URLs just to keep the code we provide tidy and compact, but it works fine either way.