It wouldnt let me lay down the full barrier, only two tiles and then not offer me the third as option on that square i marked. Only theory i got is that the new barrier somehow was affected by the old one, which was mine too though.
Apparently, it does not matter whether there is a barrier yet or not, you just cannot create an wall diagonnally across the square you are on.
This happened to me once in the Summoning Race PvP map. I was very confused - this looks very similar.
Somebody Want to test this with wall of fire, too?....just for consistency. (I doubt you'd lava yourself, but never know.)
This is the answer. Diagonally across the caster is the only configuration of a 3 tile wall, such that the caster cannot turn and face every chosen tile. If you watch when you cast a wall spell, it turns the caster if you place a tile behind the caster. The entire wall is always in sight of caster after the turn. Diagonal wall across caster would not allow the to happen.