RE: XPS 9550: Latest Thunderbolt firmware upgrade destroys HDMI port
Same problem. Hdmi port just flickers the screen on and off. DisplayPort screen through thunderbolt port still working.
XPS 15 9550
Installed the Intel_TBT3_FW_UPDATE_NVM26_0RX1K_A07_4.26.03.001 update
It is not possible to downgrade the firmware with any old installer as it says it is fully up to date.