The PayPal error is confirmed and I am looking into that. There have been no changes made to the forum script, so it looks as if it might be a PayPal issue. As mentioned, I'm trying to find an answer.
The problem with the invisible donation amounts is also confirmed. There are two ways around the problem -
1) Hover your mouse over the jQuery donation menu, hold down the left mouse button and highlight the text in the menu.
2) Scroll to the bottom-left corner of the page, click on the DXF-Red link and in the resulting jQuery menu, select the Default style.
This is a problem that could be re-coded, to fix it in the DXF-Red and DXF styles, but then the problem would exist in the Default style. The developer of the Donation Manager add-on has been promising to look into the matter, but has been dragging his feet with it. There are some external and internal problems at XenForo, which have led most add-on devs to adopt a wait-and-see attitude. It's not been a big plus for XenForo license holders, but that's where we are, just the now.