I'm not very good at rabbit stuff, as I'm a new rabbit owner.
I did find a thread on a forum, where allotment owners were discussing things that wild rabbits ate & what they wouldn't. This was one of the plants that they ate. On the Rabbit Talk forum someone said that it is also known as goat's beard, and they often fed it to their rabbits. It is also listed on several human foraging sites.
That said is also featured on a thread listed as a plant that wild rabbits don't eat.
There doesn't seem to be a definite answer, but it certainly doesn't feature on any poisonous lists that I've seen.
Sorry I couldn't be any more help, my Google-fu must be failing me today.