The two are unrelated. "guess at" means "estimate". I'm going to guess at how much that bag weights = I'm going to estimate how much that bag weights. "guess riddles" makes no sense and you need to reword it to get any kind of meaning. 'riddle' can have a few meanings. One meaning is 'puzzle', so you could say "I guessed the riddle incorrectly" = "My response/answer to the puzzle was incorrect"