R5 and R shutting down when zooming adapted 24-70 2.8L II

My R series cameras seem to shut down when using the adapted EF 24-70 2.8L II lens. The other day I got the dreaded "communication error" with the lens, it went away for a while and yesterday the camera just went dead in the middle of shooting. The top display stayed on, but even powering it down and back on didn't clear it. I had to remove the battery and turn it back on.

The issue only seems to happen when I zoom IN. If I turn the camera on with the zoom at 70mm, I can shoot fine, and as I zoom back to 24mm I can shoot no problem. The second I zoom in, though, the whole thing goes black and the camera is unresponsive. This is happening on my R5 and R, and I have two different Canon RF mount adapters connected. I tried the lens with my 5D IV, and there doesn't seem to be any issue. Updated the firmware on the R5, and that didn't help. Is it just a matter of cleaning the contacts better on the lens? Clearly something is happening in the adapter connection, right? Any help very much appreciated.

Well, I can confirm that's not normal behavior for the R / R6 + 24-70 2.8L II with the standard, no frills Canon adapter, but I don't think that's news to you!

You say the lens seems fine on your 5D4, but just for completeness have you tried testing it in liveview? Other than that, I would definitely try cleaning the lens contacts to see if that helps at all.

I take it you have other EF mount lenses and they all work fine on your R bodies via the adapters?

The closest I've had to it was a third party lens (Tamron 24-70 G2) that seemed OK when used through the OVF but when in liveview suffered from rapidly fluctuating aperture (despite being set wide open), followed about 30 secs later by a comms error. Needless to say the lens went back and I ended up getting a nice used 24-70 2.8L II which has been utterly dependable.

Fingers crossed for a painless fix from Canon. I'd be very interested to hear what they say about the nature of the fault.