Steps to reproduce:

1. Open the following process in DEV setup: http://localhost:8080/camunda/app/cockpit/default/#/process-instance/113c5b16-ed8c-11e4-bd89-56847afe9799/runtime
2. On the "User Task" select "start before".
3. In the modification instruction, select the following Ancestor: processWithSubProcess

=> when executing the modification an exception occurs.

Cause of the problem:

In the modification request, the activitiy Id is sent to the server instead of the activitiy instance id.

Notes:

1. this may happen whenever I select the "root" of the tree (aka. the process instance itself) as ancestor.
2. unrelated: should the process instance be selected by default?