The Xbox uses an ethernet Physical Layer chip that might have holes in its cable length capabilities, specifically in their adaptive equalizer. I’ve seen the errata posted for those chips.
Test using 10BaseT on your router, don’t use the Xbox. If you can send data back & forth to your router, the cable is good. 10Mbit is more robust than 100Mbit ethernet.
If the cable works but not at the length you’re using, it will be cheaper to start cutting a few meters of cable and refitting the RJ45 — you were just going to throw the cable away, right?. Make sure you retest again on the router.
Never realized that smart move on the mfrs part.....