Chosen Solution
After waking my Mabook Pro up from Sleep mode, it showed a flashing question mark on the startup screen and didn’t boot. I forced a shutdown by pushing the power button and rebooted the computer. Now the machine started up again but the internal keyboard and trackpad stopped working. An external keyboard and mouse still work. Even after several reboots, the internal keyboard and trackpad never worked. When logged in, in 90% of the cases the bluetooth symbol is crossed out and says “Not available”. In about 10% of the cases the bluetooth works like normal though. There was no water damage or impact, which could have caused this issue. The battery has been replaced ~5 months ago, within these 5 months I have never experienced problems. Edit: I took out the logic board, the rear looks fine besides some dust (photos attached). There is no corrosion damage. Also unplugged and plugged the trackpad cable both from the logic board and the trackpad. After putting everything back together the problem still persists, so unfortunately not just a lose connection.
@gerdputzl - Did you have some liquid spill into the keyboard or trackpad at some point? Your symptoms sounds like U2700 chip is failing or the signals are being interfered with by corrosion damage. The Bluetooth services of your AirPort board, the internal keyboard & trackpad connect to this chip which is the internal USB hub. You’ll need to take the logic board out to inspect it as its on the back side near the cut out for the fan. If you see anything take a good sharp picture so we can see Adding images to an existing question Update (06/14/2018) I found your problem! You have quite a bit of corrosion around the I/O cable connector. This explains why your internal USB connections are not working and have a flakey connection to the SSD as the corrosion is interfering with the signals. You might be able to clean the corrosion off the logic board its self but you likely need to replace the connector as it looks to far gone. Compare to the image here, note the finish of the contacts are shiny and not stained.
Use a soft tooth brush with isopropyl alcohol to carefully scrub the area rinse it off and post a fresh image of just this area. You’ll need to also try cleaning the internal connector part and will need to replace the I/O cable as well as its likely damaged too.
Hold down CMD+V and if it stopped loading, get the log file where it stopped. That is the easiest way to diagnose without special tools. It is called verbose mode. The suspicion is the I/O cable may be loose or do not have good contact point or the Broadcom card is failing soon.
