x-api-user parameter stopped working

Our account has a subaccount.
It was used for sending documents for eSign using the x-api-user parameter for years, but on October 4th this stops working, returning the next error:
{"code":"INVALID_USER","message":"Invalid user ID or email provided in x-api-user header"}
Meanwhile, sending documents for eSign for the master account (without passing x-api-user) works fine.
No code changes were done in that period.
Any ideas?
Thanks for the help!