sort_order is a system data field. System data fields must not be changed

Hi community,

I try to change the label of the sort_order property in one of my relationship ItemTypes.

I have done this kind of change in many ItemTypes before and it always worked as expected. Typically I replace the sort_order label with something more readable for the user. 

But in my current sort_order property, I literally can change nothing, not even the label. I always get the following error message: 

sort_order is a system data field. System data fields must not be changed

I even get the error message if I don´t change anything and just click "save". In every other ItemType, changing the sort_order property works without trouble. But my current ItemTypes just won´t let me do it.

What can be the reason for this behavior? How does Innovator decide if a property is a system property?
On SQL the shown "sort_order" looks like all the other "sort_order" properties in the database. I don´t see anything difference to all the other sort_order entries.

It really would be a big help if someone has an idea what can cause this behavior!

Many thanks in advance!

Angela