Can you show us the dialogue box after clicking "preview", o_e_l_e_o must be correct that it's trying to use multiple inputs in that transaction.
Problem is: the slider was already set to the lowest possible and that indicates that it's definitely have a very huge number of small inputs.
If you try to edit fees manually to the lowest possible amount, it might stuck as long as the "mempool congestion".
I suggest you to sort out your inputs by showing the coins tab (View->Show Coins) and freeze the very small ones (v3.3.5, addresses only for lower version) or just multi-select (ctrl+left click) the huge UTXO (max 10, better if 1-2) then right click, then select "spend" to save fees.
If you try to edit fees manually to the lowest possible amount, it might stuck as long as the "mempool congestion".
I suggest you to sort out your inputs by showing the coins tab (View->Show Coins) and freeze the very small ones (v3.3.5, addresses only for lower version) or just multi-select (ctrl+left click) the huge UTXO (max 10, better if 1-2) then right click, then select "spend" to save fees.