PayPal is notorious for displaying meaningless errors for just about everything, even when it is intentional. Might not be the case here, but they definitely do things like this.
Though if I had to put my money on it, if it isn’t a bug it’s an anti fraud measure of some sort.
Just as an aside, donating does not have to be monetary. Helping (if possible) with development, triaging issues or helping with support/forums of any kind, along with in general filing good issues goes miles for FOSS projects, especially larger ones with only a few developers.