So it looks like this should fit in with the issues others are having...
Made a reservation last week for a car that I will pick up next week. Just want to extend the return time by a few hours which will not change the price as I'm already paying for a full extra day. Using a web browser since I can't get this far on the app, but when I make the return time change and click submit, I get:

• INVALID CREDIT CARD NUMBER [DE229]

Interestingly, I'm using the same credit card when editing my itinerary as I used when making the original reservation, and the credit card is already in my profile.
Do I have to cancel the original reservation, then make a new reservation?
Thanks.

Edit: I should note that I've had the same error on multiple web browsers: Chrome (at home) and Bing (yes, my work makes us use Bing).