This is caused by a bug in the new Flash version that Adobe refuses to fix. There also is a bugreport on Launchpad that discusses this (LP: #967091). The bug only appears when you use the NVidia binary drivers and have libvdpau installed. A quick fix is adding some extra settings to the Flash configuration.
sudo mkdir /etc/adobe
echo -e “EnableLinuxHWVideoDecode=1\nOverrideGPUValidation=true” | sudo tee /etc/adobe/mms.cfg > /dev/null
This fixes the problem while still having VDPAU hardware accelleration. Unfortunately there probably won’t be a new Flash version to fix this without this manual config-hack because the Flash version that introduced this bug is also the last Flash version Adobe will ever release for Linux.