[Solved] Trouble performing BLE DFU update on Bangle.js

For a fraction of second I can read on the watch screen DFU STARTED

Had you pressed any buttons after that point? Pressing BTN1 causes it to exit and run the watch firmware again which is as you described, but if no buttons are pressed it should sit there and be fine.

Maybe you could record a video to show what happens?

when connecting to the watch using nRF connect on my smartphone I get this error

That's because you have to connect when the watch is in DFU mode

Did someone successfully managed to perform a BLE DFU update with his Bangle.js?

yes, I know there are a bunch of people (me included) that do it often. The Espruino firmware was actually loaded onto your watch wirelessly via that exact bootloader so I'd be pretty suprised if it was broken.

Is yours a Nodeconf Bangle.js?

Is there a special screen for the watch that mentions that the watch entered successfully DFU mode?

yes, it says something like:

DFU STARTED
BTN1 = REBOOT