Error while fetching ParseUsers

[SilviaS]

Hi,
I have an issue when I fetch a ParseUser whose reference is stored inside another ParseObject. That is, when I perform:

parseObject.getUser().fetchIfNeeded();

the library raises the Exception "com.parse.ParseRequest$ParseRequestException: Object not
found", even if the user exists in the database and the reference inside the object is correct!
I'm using the version of the Parse library 1.13.0 for an Android app and I have the parse-server-example running on Heroku with mLab as the backend database.
Is there something that I can do for solving this problem?

Any help is welcome!