Hi Christian,
We are using the same BAPI for setting the status of a task followed by BAPI_CPROJECTS_COMMIT_WORK' and it works all fine. Check the sample code below:
DATA: LS_BUSINESS_TRANSACTION TYPE BAPI_TS_BUSINESS_TRANSACTION.
LS_BUSINESS_TRANSACTION-BUSINESS_TRANSACTION = 'CANCEL'
CALL FUNCTION 'BAPI_BUS2175_SET_STATUS'
EXPORTING
TASK_GUID = TASKGUID
IS_BUSINESS_TRANSACTION = LS_BUSINESS_TRANSACTION
TABLES
RETURN = LT_RETURN.
* check whether the return tabel contains any error entry i.e Return-Type = 'E'
CALL FUNCTION 'BAPI_CPROJECTS_COMMIT_WORK'
TABLES
RETURN = LT_RETURN.
If you are using for Project level, make sure that the project structure is created before going for these BAPI calls.