Sadly my 24-105 F4L developed this same fault over the weekend, I took it to Wex yesterday & explained to them about it being fine at 24MM but as you zoom out and use different apertures, it shows the Err 1 fault, also every so often it does look darker in the viewfinder, but then after about a minute there is a dodgy sounding click and it is light again. While they will be sending it to Fixation to be repaired, I really do hope that this fault is covered under the 1 year warranty on used products that I got with WEX, as I only bought the lens 2 months ago.