I encountered the same issue, while attempting to send in answers to EarthCaches.
I tried to use the "Message this Owner" link from the listing, as I usually do. The message didn't want to send. Also, there was not the possibility to add a picture to my message.
I resolved my problem by first visiting the Owner's profile and there clicking the "Send Message" link. This way it worked just fine (and also the possibility to add a picture).