While setting up a second bridge for virtual machines on an Ubuntu server, I ended up with this error:
# ifup br1
Waiting for br1 to get ready (MAXWAIT is 3 seconds).
RTNETLINK answers: File exists
Failed to bring up br1.
I found lots of different explanations and possibilities described, but in my case it was simply that when I copied the previous bridge definition, I had kept the gateway line. It seems you can only have one gateway defined in your interfaces file, so by removing that it all works for me now.
Hopefully somebody finds this and it turns out to be the same fix they need.