Loose connection on circuit. Toaster is a high current device and causes an otherwise normal connection to go open. Could be anywhere on the circuit.

Thanks for your advice. There is only one place where there could be a loose connection and that is in the panel. I will have a look. Have noticed other connections to be not as tight as expected.

"There is only one place where there could be a loose connection and that is in the panel."

Or ANY connection upstream of the outlet in question.

It is pretty rare to have only a single duplex receptacle on a kitchen counter circuit.

You might want to pull all the receptacles in the circuit and make sure all screws and wire nuts are tight.