Happend at the first boss Quintus Verres.
The bug is nothing other than a graphical glitch.
While Quitus Verres during first phase threw a firepatch beneath my feet, he also used a "firespirit" or whatever it is, to fear me. I got hit by both at the same time and apperently I got stuck with a fire always appearing at my feet. My friends in party could also see the flames and it persisted even through death. It disappeared once I teleported out of the instance.
We are 3 guildmates in the video at lvl 28, 36 and VR2. Running through the dungeon to complete the quest for skillpoint.
https://www.youtube.com/watch?v=RREUpK9y0kI