Hi Heike,
If while activating data there is an error in monitor then it means that your data is not activated and is not avialable for reporting. You first have to solve the activation error as said by experts.
Can you let us know if you are extracting from standard datasource or is it a generic datasource?
Sometimes the error is of invalid data during activation - error may say Invalid characters or something.
Can you paste a screenshot of error?
Thanks
Amit