Hi, I am having a very similar problem, just checked my site after a perfectly functioning distribution being in place for years and it has suddenly stopped working.
When the CDN is disabled the site returns to normality instantly.
I have tried the usual steps of updating codes etc, but I still have no success. Oddly the plugin says "Test Passed" when I try to test the distribution.
Alarmingly I have tried checking direct links to individual previously uploaded images files and they display only 20% of the time. For the record my distribution was created through Amazon and has been in place for years.