I've been chasing this ghost for a past 2 weeks now. I own a road bike with Ultegra di2 and rear shifting stopped working (kind of) 2 weeks ago. You press lever, lights on junction box illuminates indicating battery life left, but nothing happens. I started charging it, afterwards it didn't work but then randomly it started to shift like nothing is wrong. I rode 100km in the evening, went to sleep, woke up the next day- doesn't shift once again.
At this point I call my mechanic and schedule an appointment, I put my bike in my car, drive there, take it out, bring it to a shop, I start telling what is the problem and it shifts. I click like 10 more times and it shifts flawlessly. Guy looks at me like wtf, but we agree to keep bike at his shop for a day, maybe it will stop working. Spoiler alert- it didn't, worked all day, I bring it back home, ride another 80kms on it, go to sleep, next day it doesn't work anymore.
Anyone seen something like this? It seems that it stops working only if you leave it unattended for like 2+ hours. If you keep it in the room and keep fiddling with shifters once every couple of minutes- it never fails. I've been away for the past 3 days and I left it not working, I came back today and it still didn't work, but I cycled like 7kms with a single gear to my friend's house, moved my saddle and pedals onto his spare bike and boom, my bike started to shift. After 2 hours bike ride once we came back, it was not working once again. I put it in a car- it works again. WTF?
Like a month ago, I had a crash in a race, I fell on right side and broke off rear derailleur hanger, I got it changed. Battery was replaced ~year ago.
If it doesn't work and I connect it to Etube, it recognises every component including rear derailleur, although I don't have sufficient cable to run error checks. Well at least I cannot do that, you click on error check and nothing happens.
At this point in time I am a little bit desperate. I cannot take it to mechanic, because after car ride it starts working out of the blue.