Both Speakon and IEC plugs are perfectly well seated. The amp was connected to an extension lead with nothing else running from it…Try twisting off and on again the Speakon connection from the speaker in the back to the amp. It could be loose or misaligned, somehow.
Make sure the power cable into the back of the amp isn't loose.
Otherwise, there's nothing about playing backing tracks through it that should trigger shutdown like that.
If it still does it, can you try a different electrical circuit? Does the one you're using now have any kind of appliances hooked to it?