I've tried this in both 9.2.7 and 10.2.2
When splitting on Level 2 I get this error
Cause
Exception Request Unique ID: 40b304e7-9fed-448f-9152-05e786253619
When splitting on Level 1 I first got "Can't split because the document contains no header information" even though Confluence previewed the correct headers.

On my fourth atempt the Level 1 import succeeded. Level 2 still fails.
Also The link to support request form goes to a /admin/raisesupportrequest.action URL that is non-existant in my installation.