It seems that there is an entity named 'String' is that correct? That probably causes conflicts with java native implementation of String (java.lang.String). I
Suggest renaming the entity, but you can also file a bug report that proxies should generate methods that return java.lang.X rather than X for non-primitive results; that will avoid similar problems in the future.
Probably a naming conflict between your entities and MxModelReflection. Try renaming your entities. If this is the cause submit it as bugs because Mendix should warn you about the conflicts.