That behaviour has been reported before.
All reported flashing issues involved usb3 in some form and the advise to either use usb2 port or a usb2 hub solved all cases i know off.

@Fellfrosch,
Either it is usb3 problem, or if you got deeper trouble from the devel-update, revert to factory state using the Factory-Restore Tool maintained by tdm.

Viel Erfolg!

@taixzo,
Sorry to hear and it will not help you when i tell you that pebble connection should be possible in testing...
EDIT, just tested and it works on my end, but:
I had to restart the rockpool service from within rockpool dev-tools while it was connected shortly to not let it disconnect after some seconds. After that first hickup it connects reliably now "even" after reboot.