Oww i see, I read that CAN1 was tested and ok.

while going through the code, the problem is indeed not a problem, just a (certainly) test feature in the
Code:
FCTP_FUNC void FCTP_OPT::writeTxMailbox(uint8_t mb_num, const CAN_message_t &msg)
function.

Changing the size and message data allocation to the actual values fixes everything !

Note : the data registers (FLEXCANb_MBn_WORD0 and FLEXCANb_MBn_WORD1) were masked ( |= ), don't forget to reset them at some point...

Thanks for the fast reply !