Hello Felix, did you manage to solve this issue at all as i have the same error.

Hi, 

did you try in a new DB ?

If yes and still get the error, you have to do some debugging in order to know the problem. 
first, target where in code you have the message. You will notice that's inside function "load_new_partners"

Then, add console.log(..) to print values of variable "partners" and "self.db.add_partners(partners)"

you may also want to debug add_partners method to see what it does.
Obviously,  self.db.add_partners(partners) is False so odoo displays message "Failed in updating partners."

hope this helps.