about 3 out of 6 tasks done had a validation error.

Your validation rate already looks better now. A certain quota of invalids is to be expected and due to the stochastic nature of neural network learning. The admin John Clemens is already aware of this issue being caused mainly due to NaN errors in midst of the training process and will roll out a fix to periodically check the nature of the computed network weights to catch any NaN weight values and revert/reinitialize the weights to avoid some of the otherwise invalid results.