Error is seen when trying to edit the password of a Workflow Credential saved on the Gallery:
Credentials could not be updated
Invalid Username or Password
This is a known defect that is being reviewed by Alteryx.
The update is getting a POST error 405 methods not allowed when trying to use the request URL http://mygallery/gallery/api/credential/xxx.
The client can delete the user credential and re-add it, with the updated password.