I managed to find what caused the battery drain in sleep mode with help of the powercfg tool, but it took many hours of trial-and-error before I found it. It was caused by a setting I got a tip from in this thread. After I reset that setting to default the tablet hardly consumes any battery at all in sleep mode. I got another problem after uninstalling and later reinstalling the KB2822241 from Windows Update for troubleshooting the battery drain problem. Sleep mode would not be activated when closing the lid on my tablet+dock. Also the default action for the power button changed from sleep to turn off. Resetting the power scheme settings to default fixed those problems. Now my tablet works great!