I found the solution.
The unlock option is only available if IPS and IS are both updated to the latest SP:
- IPS version 4.1 SP5 with IS version 4.2 SP05 --> no unlock option
- IPS version 4.1 SP6 with IS version 4.2 SP04 --> no unlock option
- IPS version 4.1 SP6 with IS version 4.2 SP05 --> unlock option available
