This issue is reported at flatlib‘s Issues page, but the solution is not googleable, so here it is. Suggested by joaoventura.

The problem is with flatlib.ephem.swe sweObject() method, it does not cooperate with swisseph‘s calc_ut() method any more.

To solve it, you need version 2.00.00-2 of pyswisseph, and not 2.0x-something you automatically install with flatlib.

pip uninstall pyswisspeh
pip install pyswisseph==2.00.00-2