Chosen Solution
After I swaped the battery my Phone won´t turn on anymore. The weird thing is that different things happen with the old and new one installed. The new one makes it show the low battery graphic and the old one makes it show the apple logo. I reseated the connector of both batteries multiple times. After the on of the graphics flash, the lcd backlight stays on. Hard resetting or holding down the power button turns of the lcd backlight. Trying to power up after that just starts the same process again.
The software on the phone somehow got corrupt from old battery most likely. Keep the new battery connected and try doing an update via 3utools or iTunes. If error 26 is displayed after attempting update the iPhone must be restored which also does a factory reset. Only way to get data is from a prior backup such as from iTunes or ICloud backups.
