I cast a spell "Fell". The game allowed me to cast it but when I clicked on the opponent's Glissa Sunslayer to target it, nothing happened. The game would not allow me to select the creature as a target. Something very similar to this happened in the game I played immediately before this one.

In the last game, when my opponent went to attack me with multiple creatures for some reason the knights and soldiers (they were playing a white deck) were unblockable. Whenever the opponent attacked, I could not assign blockers to soldiers/knights because it would allow me to select them.