How to fix iTunes could not connect to this iPhone (or iPad) because an unknown error occurred (0xE8000015) (Nothing else worked)

Cables are the most susceptible accessories when it comes to damages. That's why they are most sought after on the market. No matter how carefully you keep it, over time it will loosen up, and one of the cords inside would detach.

Maybe the USB (lightning) cable is the causer of the unknown error 0xe8000015 in iTunes. To test this, borrow another lightning cable and connect your device to your computer with it. If you don't see the error message, and your device functions as expected, replace the cable.

Note: Make sure to buy an original cable sold by an authorized reseller, as fake cables cause nothing but trouble, especially when it comes to data transfer.

If none of the other solutions worked, then maybe it's just your computer that won't accept your iOS device, although this would be an extremely rare case.

What you can try is connecting your device to another computer. If you didn't get the iTunes error 0xe8000015, then your computer was the culprit all the time.

Consider restoring your device through DFU (Device Firmware Update) mode as one of the last-resort options to fix this unknown error, as it involves data loss. Restore it only if you confirmed that there is nothing wrong with the USB cable and the computer.

Follow the steps below, on the process:

1. Put your device into DFU mode - feel free to check this article on Apple, if you are not sure how;
2. Connect your device with your computer, via the USB cable;
3. Restore your device, following the on-screen instruction on your computer.
   
   

Note: If you use a laptop, ensure that its battery is full, or use a charger, as interrupting this process can damage your iOS device.