Hi Nicholas,
Welcome to the MagicLeap developer forums! Sorry about the trouble building Unreal! There was a change that was committed to the 5.3-release-ml2 branch last week to change the OpenXR headers that would most likely cause the break. The change was just reverted earlier today- https://github.com/MagicLeapUnreal/UnrealEngine/commit/08ac95ed90e971cc849bf76d43b287ec259154dc
Sorry about that! If you run git pull, you should pickup the commit to revert the change to the headers. You should be able to build after that. Please let us know if you still have any trouble.
Best,
Adam