I was able to reproduce this problem on my stm32f446 dev board. It seems the original CAN2 support on stm32f4 was only ever “stubbed out”. Actually using the CAN2 hardware on stm32f4 requires the software to program a goofy mix of CAN1 and CAN2.
I put together a PR to support CAN2 at: stm32: Fix CAN2 handling in can.c by KevinOConnor · Pull Request #5988 · Klipper3d/klipper · GitHub
@bondus - FYI.
-Kevin