A quick fix is to run this command:
sudo brctl setageing br0 0
This causes the aging of the MAC address to time out immediately or in 0 seconds, which delete the entry frmo the FDB (Forwarding Database) and causes the error to go away. The default time is 300 seconds or 5 minutes.
You can also add it under your br0 definition like this in /etc/network/interfaces to make it permanent and automatic:
auto br........