"Problem Powerline" flash code appears when I try to charge a particular XRP battery in any of my three charges yet all my other batteries charge fine.

It sounds like you have a defective battery. I would suggest contacting one of our Factory Service Center's to see if your battery is still within warranty. Please see the link provided. You can also give as a call at 1-800-4DEWALT. DEWALT com

This sounds like your charger will most likely will need to be replaced, in this day in time most charger are throw aways.

Depending on how old or more to the point how many times you have already charged the battery, it may be worn out. All rechargeable batteries do reach a point where they won't charge up anymore. If you have fully discharged the battery and tried to fully charge it up again and it won't take a charge or only lasts a few minutes on a charge it may be time to recycle it and buy a new one.

battery is bad and needs to be replaced. But first you might try and clean the contacts.

Could be either the charger OR the battery. Before spendin anything on a replacement try cleaning the battery and charger terminals with a Q-Tip moistened with alcohol. Even a small buildup of dust can cause the fault.

If you keep trying to charge it, it might do so. If this pack is still under warranty, get it replaced. If it is on your dollar, you can get the pack rebuilt with new cells for roughly $40 to $50 at: www.primecell.com
I had a Skill drill pack rebuilt with Lithium ion cells that have half again the original charge longevity. Hope this helps!

the battery is bad, the fast flashing red indicated an internal short in the battery or it is consumed and needs to be replaced.

You may want to try a different charger base. Any way you can borrow one so you don't have to purchase first?
If for some reason the charger isn't working well, you may be getting a partial charge and only that.

You may even want to rent a cordless just to try charging your own battery on a different base.
before you buy.

buy a new one