The source of problem was found. It was related to the handlers from Systick, SVC and pendSV. I should use the handlers from FreeRTOS instead of the functions used by the ST. So, in my source code, I changed the l vector table (on file startup_stm32f767zitx.s) to these:

.word  vPortSVCHandler
.word  xPortPendSVHandler
.word  xPortSysTickHandler

After doing this, the FreeRTOS started working.