Alexandra,
Your Camera is supported. I use a Canon 1Ds mk III & 40D. They usually work perfectly.
However, I too have seen the Red Screen with the message "unsupported file format", although rarely.
What is even more confusing is that the only times I get it is in the middle of an edit session. I'll import a shoot (500-1200 files) and begin editing. Everything will work fine for a considerable amount of time. I click through images editing each as I go. Then, quite randomly I'll click on the next image and I get the Bright red screen with the "unsupported file format" message. What is interesting is that I can go back to other images without a problem, but if I go forward to that next image I get the red screen.
My workaround has been to quit Aperture, and relaunch. In every case I am able to pick up where I left off and view and edit the image that (30 seconds ago) had been "Unsupported"
I don't know why it happens, and the problem never persists. It is intermittent, and I have been unable to "cause" it to occur. For me it has just been an annoyance, ---I've lost no work and it has "only" happened perhaps 25 times in a year of nearly daily use.
I do not have a solution,but I have offered my experience in the hope that additional information might lead to some help for both of us.