Not so brave, as i have nothing to lose if the disc is not compatible ^^.
I tried a sony single layer media (SONY-NN3-002) and it worked fine, no problem with the writing. I didn't found any DL madia as of now in my town to test.
Seems to be a compatibility problem (tried burn4free and nero linux, no luck either). Weird as the VERMATIMd appears in the compatibility list of the burner.
I contacted sony hotline and they ordered the Verbatim discs to perform tests. They'll let me know the results in a week or two. We will know if the discs should really be compatible or no, or if my batch could be in cause (or the burner ?).
I'll post the results here too, as it could help people with same drive.