External Flash Loader don't work

I'd imagine its interfering with the peripherals, pins or interrupts.

Couple of things you might try.

You could add a delay spin-loop in the Reset_Handler to give the debugger some time to take control.

You could redirect the vector table SCB->VTOR in to the ROM

Disable SysTick or other interrupt sources at the peripherals.

Check that your code properly resets the QSPI/OCTOSPI peripheral, and things are in weird/non-reset conditions.

You can use a UART to report internal state, and perhaps learn something about the state of things in the working vs non-working cases.

As Tesla DeLorean (Community Member) mentioned: When internal flash is empty, the CPU will run into hard fault and halt right after reset. As soon as you've programmed the internal flash, the CPU will start initialization of peripherals, RAM etc. which certainly will interfere with the external loader.

So, if you don't want to touch your application (delay after reset), the debugger must connect under reset and halt the CPU right out of reset. And, of course, beware of watchdogs: If automatic activation after reset is enabled ...