For reference below is pasted Canon care explanation of what this icon means.
It looks like perhaps there is a problem with the TTL circuitry. Once I figure it out I ll get back to you.
Now that you have answered where the icon is located im going to tell
you exactly what it means. This icon is displayed when the flash head is
not firmly snapped into place, on an EOS Speedlite when you lower the
flash head it snaps firmly into place, facing straight ahead, 90 degrees
so to speak. If the head is depressed downward, past 90 degrees, the
flash symbol flashes as a warning that it is not locked firmly into
place. The negative 3 means that the exposure compensation is set to
minus 3. The three flashing lines radiating out from the flash head
means the flash head is active. When using a 580 ex II it is possible to
disable the flash head and just use the infrared beam for autofocus
assist,or when you just dont want the main flash tube to fire, there is
also an 'active' symbol for that as well. Its shaped like a zigzagged
arrow.
So theres your answer. Thats what the symbol means, it flashes
simultaneously on the flash panel as well.
As to whats causing it? We have no clue since you are not using a Canon
EOS Speedlite flash, its possible there is some communication issue
between the camera and the flash. Its getting some false signal from the
flash. There no telling at this point. I understand you have used it
prior to this issue with no problems. However, there is nothing I can
recommend to make it go away. Perhaps the makers of the Inon Strobes may
be able to shed some light on this.
