Great! Thanks for the help. Sorry about being vague, I came across this assertion error during training and now it’s gone.
Added an assert on targets (and to catch the assertion from binary_cross_entropy), I’ll let you know if any progress is made on this issue. One thing on the back of my mind is: if my targets are created by the same function during training, is it possible for it to become invalid on epoch 5 after being fine in the first 4 epochs?