Hi Alejandro,
I would suggest you to please delete this service interface and create new one,this might solve your issue.
Because i have also faced certain error for which no resolution could be found ,but delete and created new and it worked fine.
So i will suggest that if this problem is only related to this interface then go ahead and delete and create new,Or if this is common problem then you can try for:
1.restart java engine
2.full cache refresh
3.pi server restart.
Regards
Naveen