Never had dx11 launch issues until the day 5.5 launched. I tried everything. What finally worked for me was a combination of reinstalling the game and opening the ffXIV.cfg file and changing the Screenmode from "0" to "2" for borderless windowed.

After changing back to windowed mode and shutting down my pc and rebooting this morning, i got Dx11 errors again. Changing the .dfg file to be Screenmode 2 for a second time fixed the launch issue.