Error: Invalid Value with Ui.loadResource() and JsonData on real watch

Hi, I'm developing a datafield and I get an error when trying to run it on a real watch. In the simulator everything works fine.
It's a specific line that causes the error:
var json = Ui.loadResource(Rez.JsonData.jsonFile)
Without this line I get no error. In the CIQ_LOG it says the following:
ERROR: Invalid Value
DETAILS:
STORE_ID: 00000000000000000000000000000000
with Callstack pointing to this line.
My jsonResource.xml looks like this:
[HTML]<jsonDataResources>
<jsonData id="jsonFile" filename="outputXPoints.json"/>
</jsonDataResources>[/HTML]

I don't know why it works in the simulator and not on a real watch. I'm using a Forerunner 235.
Maybe someone has an idea? Thanks in advance