First, welcome. This is a great place and a fountain of knowledge.
Can't help with the reader, but my 03 twin spark 1150 GSA also developed a flashing ABS light back in 2016. Brakes worked fine and no other symptons but couldn't get rid of the flashing warning light. Removing the tank and checking out the dreaded servo revealed no problems and everything seemed to be working as the manual and on line research said it should, but couldn't get the light to go out. Following an appeal on here, one of the old hands told me to check the contacts in the rear light - not the bulb, but the socket it fits into. I did and noticed they were coated with the same sort of deposits you used to get when the old ignition points (remember them?) were arcing. Scraped the contacts clean, and a quick squirt of ACF 50 to prevent further corrosion/oxidation, new bulb and 5 years on there have been no more warning lights. Of course, this may not be what's causing your problem, but it's only 2 screws to get the rear light lens off, so worth checking before you get up to your neck in spanners.