Just been doing exactly that and I found the error just as your email arrived.
And I made the most fundimental schoolboy error - the Stack. - Do I feel dumb!
While the freeRTOS stack is way big enough for the main GUI loop, I was performing the lcd/lvgl in main(), which has next to no stack (by design). Why it ever worked (with no optimisation) i’ll never know.
Thanks for the very quick response.
And now I have a (working) port for the STM32F4 using SPI and DMA for the ST7735 driver - happy to share if someone wants it.
I’ve had a working port of V5.4 for a while now, and thought it was time to upgrade.
thanks
Mark