Unable to provision with private key


I’m trying to provision using stored keys into a 7.8 environment,

First I generate my keys

$ c3 key -e https://sandbox.c3iot.com/
 What is your username? myusername
 What is your password? 
Creating User file ...
Stored user myusername in ~/.c3/c3-rsa.sandbox.c3iot.com.user
Time taken: 0m 0.708s

Then, when I try to provision with the key it fails with:

$ c3 prov tag -t tenant:tag -e https://sandbox.c3iot.com/ -k ~/.c3/c3-rsa
Error: c3-prov: unable to login with username myusername with c3Key file ~/.c3/c3-rsa. You can provision your keys using 'c3 key'. (HTTP status: 401)
Failed to create your c3authkey.

Any clue?