Unable to create mesh : Error Free Tetrahedral 1

Hi everybody,

I am having this error when I try to mesh a 3D structure :

The following features have problems :
Features : Free Tetrahedral 1


I don't find a way to have more information about this error.
My geometry is really complex, with many points/edges. I cannot make it simpler.
I still have the same error when I change the element size to be 'Extremely coarse'.

Does someone have an idea to fix this error ?

Thanks.