Not rendering specific area of the image

Hello there,

When I make a render,
every time there appears a bar (a thick line) that stays grey (non-rendered).
See attached image (the black bars I added myself).
This problem only appears when I use semi transparency,
for bot rhino as toucan materials and both renderers.
How can I solve this problem?

Thanks in advance,

Eva, can you save a simple file that uses these materials and check whether the problem shows up- if so, please post the model here or