Unfortunately I don't know what is causing this and how to fix this issue or whether that's even something we can fix on our end, but upon doing some research in the paypal forum I found that many people seem to have the same problem starting sometime around the end of last year.
I suggested work around seems to be to send the money directly to the paypal account instead of using the donation button, so you can try that and hopefully it will work? Try sending the amount you wish to donate to
[email protected], add a note with your username if that's possible (or send me a PM) and I can hand out the Fairy Coins manually.
I am sorry I don't have a better solution for you right now.
![Gloomy :mcgloom:](./images/smilies/gloomymochi.png)