Have you had a recent tire change? If a wheel sensor got yanked in any way, the intermittent connection might be right there.

 

Last tire change was already 2 years ago, so I don't think it is in the wheel sensors. Also, the ABS control light behaves normally: gets on blinking when I start the bike, and goes off as soon as I start moving.
Meanwhile I bought HD's Electrical Diagnostic Guide. As an e-book they are very affordable. Contains a lot more info on the electrical circuit than my Clymer workshop manual. Hopefully it gives me the golden hint I need for this.