Training Puzzle Incorrect Solution

Puzzle in Question: http://en.lichess.org/training/31418

There are two mates in 5 (confirmed with engine).
1.Ra3 and 1.Bd2 are both mates in 5. Training problem recognizes only 1.Ra3 and gives 1.Bd2 as an error.

You are right. Ra3, Rb3 and Bd2 are all equally good options. But phew... you saw Bd2 before Ra3/Rb3? Not bad!

Yup.