Try yer local London Camera Exchange. If they’re anything like Southampton LCE, they’ll have a “man who does” on tap. They/he fixed the same issue in my 24-105 L series for just over 200 quid. Took about three weeks.
I was surprised to read that Canon won’t repair it - I was under the impression that they committed to repair L lenses for life, though I have no idea where I heard that (and whether a sensible cost is another matter).
I got this problem sorted out via Clocktower Cameras, 32 Church St, Brighton and Hove, Brighton BN1 1RL Tel: 01273 706010. I think they sent the lens to a repairer in Uckfield, East Sussex. Might be worth giving them a call. Or you can send a message via https://clocktowercameras dot co dot uk
I may enquire directly but have read on a few sites that they no longer service/repair this lens.
A darned shame - as a hobbyist investing in reasonably expensive glass (all relative of course) it’s a bit frustrating that this is a known problem, and that the replacement presumed non-OEM ribbon cable is $3-$4 on ebay.
Started watching a video of a chap’s DIY repair - would rather let someone who does this regularly handle it - maybe 20 years ago I’d have had a go, but eyesight/dexterity are 20 years worse!
I had a 100-400L serviced by Canon UKin 2022, but it was only 2 1/2 years old. I know my brother-in-law had a couple of lenses serviced/repaired by them not many years ago, and I’m pretty sure one was a EF 24-105 f/4L IS (not the 2.8). I remember because it was a lens we had in common, and as a workhorse lens he had two so would never be without. I also recall him saying they were good, and sensibly priced (which is why I didn’t hesitate tk use them later myself). But I think his was the Mk1, and it would have been several years before COVID
You’d think if it was a known design issue after a few years (ribbon cable apparently has to effectively concertina/stretch as the lens barrel changes length for zoom causing wear and tear) they’d have stockpiled large amounts of a relatively inexpensive part, even if they charged a premium price for it down the line. I’d imagine the labour/skill replacing it would be the main cost in any event.
Sadly the newer mirrorless/more compact designs and leaps forward in mobile phone cameras for the average person have made camera/lens repairers less viable as a business.
I’ve not actually contacted Canon to ask if they can repair but read they no longer work on the Mk I version after Googling the fault (which gives a vague error telling you to clean the contact pins on the camera body).
Outside chance I’m wrong I suppose but have a few good suggestions to follow up.