Another possibility that is unique to your situation (trying to load Apache POI of all things…) is that the Reporting module already depends on an older version of POI (3.14), and since the class path is flat in the Client/Designer scopes there’s no guarantee that your POI takes priority over the POI from Reporting (or the other way around - DANGER!).
So, if you have the Reporting module installed, try uninstalling that…