After checking your transaction again, it seems like it also lacks the second part that comes after the transaction in a PSBT tx (it is all zeros 0x000000) so the parser fails to populate the txins then fails signing the transaction later one since they are empty.
https://github.com/spesmilo/electrum/blob/22e6fe09c17724cd75763306470fa46a7bb7c4e8/electrum/transaction.py#L1709

So your problem must be in the "payto" step which creates a wrong PSBT.