This has happened to me as well.
The problem seemed to be casued by removing the default value of an enumerator present in the structure I’ve made.
The solution was simply change the default Enum value in the structure then change it back to the desired one and re-save the structure.
Seems like Structures does not auto-correct/fix the default Enumerator value when changed in the enumerator. So we need to go to each structure and set it manually.
See my example below. I had to select TRIGGER TYPE to be “Generic” Cause I had one named “None” which I removed and the structure had that one as default, not the “Generic” one. So I just set it back manually. This fixed it.
Hope this helps. Happy Devving!