Xbox One X. It is not a hardware issue, unless have bought a faulty Xbox, which MS will be replacing soon?  Console is kept up-to-date, and has ample memory.

20mbps download/0.9mbps upload (on last speed test).  I have ongoing ISP investigations. Both BT (ISP) and Openreach (infrastructure network) claim there are no faults.  A new router is en route (due to Openreach deflecting responsibility) to definitely rule out hardware fault.

The problem is a Minecraft/Xbox network issue:

On opening Minecraft, game hangs for 10minutes. The background image is still (does not auto pan).

Eventually, the 'Failed to sync user data. Loading user data' message pops up.  This loading process never completes.

I was previously able to click 'cancel', see the 'loading resources' message briefly, and then play my world, but now, my skin/achievements/etc will not load.

I loaded my world despite the skin not loading, but character was invisible, and was able to pass through some solid blocks (and all controls were reversed: Down=forward, Up=backwards, Right=strafe left, Left=strafe right, and my inverted 'Y/camera/look' was no longer inverted).

Since first attempts to fix/get help, the problems are getting worse!

I have tried reloading game, hard resetting console, and uninstalling/reinstalling Minecraft.

It takes a very long time just to load to menu screen. My Xbox Live subscription is running purely for Minecraft access, but unable to play the game.

The last 1hr 48min of my life was spent trying to load a game, and typing this attempt at getting some assistance (excluding the weeks prior, that I have been enduring these issues).