Thank you for your response and troubleshooting, and you're indeed right. I guess that's what you get when you make last minute changes to a worksheet the night before. Of course the U variable is defined as both a function and a variable, and that would cause issues. Thanks again for finding the issue, a bit embarassing on my part I must admit.
I'm sorry, but I don't quite understand your earlier objection. As you can probably see from my Maple worksheet, I'm not that confident with the inner workings of Maple and syntax, and it's not unlikely that I'm doing something that is not that accepted within Maple with my cumbersome expressions. Although they do yield the expressions I expect, so I'm crossing my fingers that there won't be any more problems in the future (bar the ones caused my own stupidity).
Edit: Disregard this paragraph, I now see that this answer relates to the previous issue of multiple defined U, and not the nested differentation itself.
Thanks again for your thorough and quick responses.