libGDX Json converts Enums to strings and therefore cannot serialize / de-serialize Map keys to anything but strings. To fix this either

1. Use strings for keys in Maps (ObjectMap, HashMap ect.).
2. Use GSON which can serialize Enum Map Keys just fine
3. Write lost of special serialization / de-serialization code using a class that implements Json.Serializer where T is your map and then register that with your libGDX Json instance .

I recommend option one or two.

libGDX Json converts Enums to strings and therefore cannot serialize / de-serialize Map keys to anything but strings. 

To fix this, either...

1. Use strings for keys in Maps (ObjectMap, HashMap ect.).

2. Use GSON, which can serialize Enum Map Keys just fine.

3. Write lots of special serialization / de-serialization code using a class that implements Json.Serializer<T> where T is your map and then register that with your libGDX Json instance.

I recommend option one or two.