Could you try setting the configMAX_SYSCALL_INTERRUPT_PRIORITY to something higher, for example:
#define configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY 5
#define configMAX_SYSCALL_INTERRUPT_PRIORITY ( configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY << (8 - configPRIO_BITS) )