XC60 Key Battery Low Warning

Sounds like a classic case of a dying key fob battery, like a flashlight with a weakening bulb. Have you tried replacing the battery in the key fob yet? That's always the first thing to check. What kind of range are you getting when you try to use the remote? Does the spare key work any better?

Okay, good analogy! No, I haven't replaced the battery yet, I wanted to check here first. The range is basically zero now; I have to hold the key right next to the door handle for it to even think about working. The spare key is doing the exact same thing, which made me think it was a car issue. If it's both keys, is it still likely just the batteries?

With both keys showing the same symptoms, especially with the error code, it does strongly suggest the batteries are the issue. However, since the problem exists with both keys, consider this: a fault in the car's receiver could drain key fob batteries quicker than expected. Replacing the batteries should solve your problems, however, I would still go to a workshop, let them check the error memory and tell them that you suspect a parasitic draw. This way you will have an easy fix, but also be on the safe side.

Thanks a lot for the advice! I replaced the batteries in both key fobs, and everything is working perfectly again. Keyless entry is back, and the error message is gone. I let them check the error memory like you suggested and it indeed was just a low voltage error from the keyfob. The repair cost was around 37€, so it was indeed the batteries. I appreciate you pointing me in the right direction!