To reproduce:

1. Download attached project "ObfuscationRepro3 2.zip" and open in Unity
2. Press File > Build Settings...
3. Switch platform to Standalone, iOS or Android
4. Set Scripting Backend to IL2CPP
5. Press the Build button
6. Observe that Build fails
7. Inspect Console

Expected results: You will see a proper Stack Trace and you can backtrace what's going wrong
Actual results: You will see an unclear Stack Trace:

IL2CPP error for type 'PROJECTNAME.Exercise.ExerciseEvent' in PROJECTPATH/Assets/Libraries/MusicRepresentations/Exercise/ExerciseEvent.cs:82
Additional information: The given key was not present in the dictionary.
il2cpp.exe didn't catch exception: System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
at Unity.IL2CPP.Metadata.VTableBuilder.OverrideInterfaceMethods(Dictionary`2 interfaceOffsets, List`1 slots, Dictionary`2 overrides, Dictionary`2 overrideMap)

Notes:
- If you change Scripting Backend to Mono everything will work fine
- Building with Obfuscation disabled works fine, so this is clearly an error there, but it'd be nice to get a better error message
- Could not check on Unity 2018.1 and 2018.3.0b12 because project becomes corrupted

Reproduced on Unity 2017.4.0f1, 2017.4.16f1, 2018.2.18f1, 2018.3.0b9 and 2019.1.0a10