I took a look and I see what you mean. That validation window popping up is not part of the plugin.
When I traced where it comes from, it seems to be part of something called AnswerCloud/SiteBrains or similar. Is that something you have installed, or ring a bell?
Apparently that validation script attaches itself to any form button and tries to validate it on submission. They should probably make sure that they only attach themselves to comment form buttons (I suppose that is the purpose) and not every form/submission button available on a page.
So try disabling the plugin that adds that validation function, and you will see the PayPal plugin working.
From the PayPal Donations plugin's point of view, I can't do anything to override what that validation plugin attaches to the button. It's out of my control. But that AnswerCloud/Sitebrain script could probably be updated to verify that they only attach themselves to the buttons they are supposed to validate.
Hope this helps.