BAPI_BUS2175_RELATION_ADD Error occurred when creating the relationship

Hello experts,

I am getting an error by creating a relationship between two tasks with BAPI_BUS2175_RELATION_ADD.

The error is: E DPR_BAPI 149 Error occurred when creating the relationship

The task id's I use are below the same phase. Following inputs i make:

TASK_GUID <task_guid from predecessor>

IV_RELATED_TASK_GUID <task_guid from successor>

IS_RELATIONSHIP

TIME_INTERVAL 0

CALENDAR <empty>

RELATIONSHIP_TYPE 1

IS_RELATIONSHIP_UPD

TIME_INTERVAL X

CALENDAR X

RELATIONSHIP_TYPE X

Can anyone help me, what I'm doing wrong?

Thanks.

Regards

Christian