Error when updating case variable

Hello,

I try to update caseVariable using the Rest Api "/caseVariable/" + caseId + "/" + variableName

When I call the get methode. I receive the variable correctly. But I can't update it, i always receive the same error even if I change the user.

this errors

"{\"exception\":\"class org.bonitasoft.web.toolkit.client.common.exception.api.APIException\",\"message\":\"Error when updating case variable\",\"cause\":{\"exception\":\"class org.bonitasoft.engine.exception.UpdateException\",\"message\":\"USERNAME=Sam | org.bonitasoft.engine.core.process.instance.api.exceptions.SProcessInstanceNotFoundException: Process instance with id \\u003c1002\\u003e not found\"},\"api\":\"bpm\",\"resource\":\"caseVariable\"}"

Pls help me