Error 14 During Update

A few days ago I updated my iPhone 5s from iOS 11.x to 12 using iTunes. Somewhere in the middle of the update it stopped with error 14. Since then the iPhone can only boot into DFU and recovery mode. In iTunes I have tried to update without restoring to no avail. It always ends in error 14. Using the Apple Configurator I tried Revice Device from DFU which also ends in error 14.

I tried the obvious things such as using a different USB cable and a different computer with the same result. Since this iPhone has a history of battery issues I removed the the old battery and connected a new one. The result stays the same. Error 14. It is the same with the screen (digitizer, front camera and home button) disconnected.

All parts but the battery are original. I still have the old weak battery but it was the first part I replaced for debugging anyway. I have not checked for damage below the 1.2 mm screw since it is the first time I disassembled it anyway.

I have not yet tried a restore which wipes /private/var as I still have some photos I have not yet backed up :-(

Any pointers?