All I had to do was to remove: mango/classes/org/jfree/data/Range.class

I've seen this problem in two other places. At least one if not both of those users had overridden a class particular to the jfree charting library, and when we updated the jfree lib to 1.0.19 this broke for them. Previously we were using 1.0.14

Check the Mango/classes/, Mango/overrides/classes/, Mango/overrides/lib/, Mango/overrides/properties/ or Mango/lib/ for an overridden .class file in a folder matching its package (org/jfree/chart/ or for a duplicate 1.0.14 jfree Jar. The other solution is to roll back to version 1.0.14 of the Jar.