Can Omaha say that I'm not an active employee because I work less than 30 hours per week?

On May 23, 2012, I gave notice to my employer that I would be resigning and would officially quit after consuming all 90 days of my accumulated personal leave and my boss concurred that I would go on leave until September 1, 2012 and my employment would be terminated at that time. On June 17, 2012, I fell off a horse and broke my back. I applied for short-term disability through my United of Omaha Life Insurance Policy. After an appeal, my claim was denied stating that I was no longer eligible as of May 23, 2012 because I was not actively employed and I must be able to prove that I worked a minimum of 30 hours per week to be considered actively employees. How is this possible? Omaha defines being “actively employed” to include “a regular paid holiday or day of vacation”. Since after May 23, 2012 I was considered on vacation using my personal leave, how can Omaha deny my claim? Any thoughts you could provide would be greatly appreciated.