doorbell2 - melted plastic around the back terminals

I bought ring doorbell2 october/november 2019I had some issues when connected to an external chime and Ring replaced the device.Now the device went offline, went to check the supply line and it was marking an output of 13v ac. Then opened the cover of the doorbell and noticed burning smell. Checked the back terminals and there was melted plastic around the back terminals. The wiring was still in place. What happened?