BCrypt::Errors::InvalidHash (invalid hash) Devise 1.5.4 Rails 2.2.5

we are using the following in devise.rb

config.encryptor = :bcrypt

We wanted to change this to

config.encryptor = :authlogic_sha512

Also written code to decrypt old password and hash it to new one when signing in (inside session controller)

However, after updating the password, it gives an error

BCrypt::Errors::InvalidHash (invalid hash):

This error comes at random places if I comment out the original point of the error. So I suppose there is something wrong with either the devise configuration OR with the user.rb model.

The user model has the following line:

devise :database_authenticatable, :registerable, :confirmable,
     :recoverable, :rememberable, :trackable, :validatable, :omniauthable, :omniauth_providers => [:auth0, :google_oauth2]

Any idea what could be the issue here? thanks in advance.