Ok, a colleague of mine (Thank you Andrew) found actually the answer and solution:
For whatever reason the Element Id of Phase 4 is “0” as this little test demonstrates:
We thought we could fix it by adding a new phase ‘after’ Phase 4 and then merging it. We thought that would give it a new ID, but for whatever reason that didn’t work. Maybe I merged them the wrong way around? I merged the new Phase with Phase 4. I should probably have merged Phase 4 with the new one.
However, he gave me also a tip for a workaround that works for me:
We can use Parameter.ParameterByName instead of Elment.GetParameterValueByName and use String.Remove to remove "Phase : " from the results.
I still have no idea what is causing that Phase’s Id to report as “0” though.