Hmm, try logging into Sythe on a different browser and then try it. It may just be you need to clear the browser's cookies and cache out, but try a different browser before you do that.
^ If this doesn't work, you are going to have to PM Matthew and ask him to help. It may be a coding or permission problem. Edit: Actually I remember The Black Tux had this problem before. It is because you have to much on one page/post. Your OP is to large to be saved, which means you are going to have to create a second thread.
I didn't use a different browser but I did it on the mobile version of sythe and for some reason it worked. I thought that might be the case but other large traders have 1k+ vouches shown in their threads so I still don't know why it happened but I used the mobile version and it worked. If the problem ever arises again I'll speak to an admin about it.