CAN mystery. Receiving messages but not able to send

Hi.

I have a program that sometimes fails to send data on CAN but still receives them correctly.

This is on an STM32L462 using 1.4.2 IDE and latest API.

For some unknown reasons, at one moment, out mailboxes never empty. It can be anytime from right upon starting, to after a few hours.

I first thought that i had a can networking cable problem, until i received data. Still messages in outboxes would not be sent.

Do you have any idea of what could cause this?

Thanks.