600EX-RT used as a remote shutter release

the setup is a ST-E3-RT on a 5DIV and a 600EX-RT as a slave. Until recently, the 600EX-RT functioned perfectly as a remote shutter release. Now, it won't. I have changed batteries, followed the manual instructions repeatedly to no avail.

The link light on the ST-E3-RT flickers red when the release button on the 600EX-RT is pressed.

The shutter release on the camera fires the slave unit as usual. I tried another 600EX-RT and it performs the same.

Is it time for the repair shop?