What it means
A transaction that reverts with the raw data 0x1231ae40 hit the custom error ETHNotAccepted, defined in Uniswap Universal Router. The explorer shows the hex because it does not have the contract's ABI to translate it.
Native ETH was sent to the router in a context where it does not accept it.
How to fix it
Rebuild the transaction from the dapp. If you crafted it manually, do not attach value for token-to-token routes.