Another proposal. Fixes the bug and logs a warning

src/caosdb/configuration.py

...onnection/authentication/external_credentials_provider.py

src/caosdb/connection/authentication/interface.py

src/caosdb/connection/authentication/keyring.py

src/caosdb/connection/authentication/pass.py

src/caosdb/connection/authentication/plain.py

src/caosdb/exceptions.py

tox.ini

unittests/test_authentication_external.py

unittests/test_authentication_keyring.py

unittests/test_authentication_pass.py

unittests/test_authentication_plain.py