Not sure if its worth a fix but I managed to repro anyways, using Blender 2.81 and exporter 6.3.1. Any armature with 2 bones (parent and child) will fail to export if parent has scale of 0 anywhere. I suppose its known and the workaround is simple but fwiw, I did not find any mention in the doc. Hope it helps.
Blend file attached. parent_bone_scale0_bug.zip (76.8 KB)
return (SystemMatrix @ matrix_world @ bpyBone.parent.matrix).inverted() @ (SystemMatrix @ matrix_world @ bpyBone.matrix)
ERROR: Matrix.invert(ed): matrix does not have an inverse