The Moralis 1-Inch plugin uses a transaction format that is too advanced for MetaMask’s default Polygon RPC node. It throws an RPC error.
Uninstalling MetaMask and re-installing it using the Moralis QuickNode allows 1Inch plug-in Tx’s to go through (say for the infinite allowance unlock funciton). I suspect the 1Inch plug-in was only tested against the Moralis SpeedyNode RPC.
That makes the Moralis 1Inch plug-in useless to users with pre-installed default Polygon networks in MetaMask (say from chainlist.org). Might want to dumb down the plug-in to reach a (much) wider audience.