JAuthentication: :__construct: Could not load authentication

Since I installed System - EasyCalcCheck PLUS - ECC+ on my Joomla 1.6 site I cannot log in anymore.

As long a s I was logged in I could access everything fine.
Once logged out logging in is not possible.
Instead I get the following message:

JAuthentication: :__construct: Could not load authentication libraries.
Username and password do not match or you do not have an account yet.

I there a way to repair this or can I forget it and need to reinstall?

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