Power Button Cable failure reported after moving Dell XPS 8930 to a new case

The power supply on my XPS 8930 gave up the ghost, so I replaced it with a Corsair CX750M (probably not relevant).

While I was at it, I also replaced the case with a Thermaltake CTE T500 (probably relevant).

I connected the power switch to pins 6 and 8 on the front panel connector, following the diagram at XPS 8930, GPU and CPU Liquid Cooler, PSU, Case Swap, Upgrade.

Things work as expected: I can power the computer on and off with the power button, all cool.

BUT: Every time the computer boots, I get an error message from SupportAssist during POST: "[...] Alert! Power Button Cable failure". I click Continue, and everything is peachy.

Does anyone know how I can get rid of this message? Did the power button in the original case know some secret handshake that the new one doesn't?

At this point, I would be OK with disabling the SupportAssist self check altogether. I don't need any SupportAssistance to know that the machine is getting a little cranky.