Followed by this entry in the images list after closing this:
You should still be able to create images via command line, but Horizon won't work.
There is a bug filed for this:
Their fix is simple: connect to the horizon container from an infrastructure node:
lxc-attach -n `lxc-ls -1 | grep horizon | tail -1`
HORIZON_IMAGES_UPLOAD_MODE = 'direct'
HORIZON_IMAGES_UPLOAD_MODE = 'remote'
After that, restart the horizon container
lxc-stop -n `lxc-ls -1 | grep horizon | tail -1`
lxc-start -n `lxc-ls -1 | grep horizon | tail -1`
And simply wait a few minutes. Image creation should work.