This strange situation came up during my last game. We just diced it off and swore to ask Portent what the real answer is:

The Evil Red Circle Character/Unit/whatever-thingy wants to charge Unit A.

He can see Unit A, and can measure a straight obstacle-free line to Unit A that is within his charge distance. The charge is therefore valid.

He is in Unit A's flank arc, so normally it would count as a flank charge.

But Unit B is sitting there blocking any straight line to Unit A's flank.

How does this get resolved? Does the charge count as a front charge, since all the red dot can see is Unit A's front? Or does he make his charge move and then just get shuffled over to Unit A's flank since he was in the flank arc?