ESP32-MINI-1 responds on the UART 1 during the programming attempt

[disobat]

I try to programm an ESP32-MINI-1 module via UART by using ESP-PROG. Unfortunately I cannot establish communication with the module via UART0. But I get in a programming attempt via UART1 to change the baud rate:

Sketch uses 290325 bytes (22%) of program storage space. Maximum is 1310720 bytes.
Global variables use 22504 bytes (6%) of dynamic memory, leaving 305176 bytes for local variables. Maximum is 327680 bytes.
esptool.py v4.2.1
Serial port COM31
Connecting....
Chip is ESP32-U4WDH (revision 3)
Features: WiFi, BT, Dual Core, 240MHz, Embedded Flash, VRef calibration in efuse, Coding Scheme None
Crystal is 40MHz
MAC: c8:f0:9e:48:3d:60
Uploading stub...
Running stub...
Stub running...
Changing baud rate to 921600

A fatal error occurred: No serial data received.
Failed uploading: uploading error: exit status 2

Enable and boot pins are set correctly. I have tried different baud rates without success. RX and TX interchanging is already excluded. Does anyone have any idea why the module can't be programmed and why it talks on the wrong UART in boot mode?
Thanks.

[ESP_LJH]

So you could not see any log through UART0? Could you measure any signal at UART0 via oscilloscope? How many boards do you test? The power supply is provided by ESP-PROG? If 3.3 V is stable?

[disobat]

I cannot see any log through UART0.
I can measure the incoming signal with oscilloscope and it does not show any abnormalities. But it is no response from ESP32.
Modules are integrated in a circuit board with some peripherals (see attached image). Three boards are tested so far.
The µC supply is stable.

Screenshot 2023-02-01 124918.png (107.19 KiB) Viewed 1335 times