Got exactly the same kind of behavior and I also have KNIME 5.3.0.
Then I tried on the version 5.3.2 and it works without doing anything else.
Apparently it worked also for this user:
Setting up Conda to use python nodes in Knime - KNIME Analytics Platform - KNIME Community Forum
Very strange however, would’ve loved it to work for 5.3.0 as it is the default one we are using.