I’m sorry but I think this needs a greater mind than mine. I can’t call that form with any set of parameters that doesn’t give exactly the same error.
If you look at the stack trace in the error, the final line :
called from [rp_zoo_session_log, line 20]
there is a multi-parent reference - it’s possible the code is confused as to where it is, but that’s clutching at straws.
Why did it used to work? Well, I know Skulpt (the client side Python interpreter) has gone through some updates recently. It’s possible that a bug got introduced (or one your code inadvertently relied on got squashed).
Sorry I can’t be more help. You might need Anvil Central on this one.
If I have a flash of inspiration, I’ll post back.