I had an external SSD drive attached via USB to the right IO board during updating. (Jetdrive could have been in there but not today 🙂 ). So if anything is connected to th board during the update it will fry the board?Most people having the issue report that they had an SD card in a card reader.
I first thought this was not the case for me, but then I noticed my Transcend Jetdrive was in a USB hub I attached to one of the USB ports :-(, so I can count myself in.
So if one wants to install Big Sure on a MBP 13 late 2013/mid 2014 (or to be sure any other model) I recommend to remove all SD cards.D
Macbook is back on running after unplugging the right IO board but i will try to reach apple support tomorrow morning again. i don't want to do the exchange myself even if that is not a big surgery.