Now I have downloaded Global firmware instead of EU firmware and it was ... sucessful 
Downgrade was also successful.
Starting phone
Connecting to the PC via USB
Starting MsmDownloadTool
Windows11 Pro:
cmd [enter]
adb devices [enter]
adb reboot edl [enter]
phone screen is black
MsmDownloadTool: Connected. Clicking Start, then downgrading successful.
Thanks for you support
(I have bought the phone in the EU, but had to use the global firmware)
Now I could unlock the bootloader.
Windows 11 Pro didn't work.
But Linux Mint 21.3 Cinnamon works:
Open Terminal.
adb devices
adb reboot bootloader
fastboot oem unlock (works)
ubport installer worked.
Thank you for your help!
Along with the phone a driver OnePlus_USB_Drivers_Setup was delivered. I have also tested this driver. But it doesn't work.
Is it possible, the Windows 11 is the problem and it has to be Windows 10?
Phone - System - USB-configuration -> allow file transfer.
Now:
adb devices -> is found
fastboot devices -> no respond
Device Manager - right click - properties:
General
Device type: Other devices
Manufacturer: Unknown
Location: Port_#0004.Hub_00001
Device status
The drivers for this device are not installed. (Code 28)
There are no compatible drivers for this device.
To find a driver for this device, click Update Driver
Driver
Android
Driver Provider: Unknown
Driver Date: Not available
Driver Version: Not available
Digital Signer: Not digitally signed
Details
Property: Device description
Value: Android
Events
Timestamp Description
6/14/2025 ... Device configured (null)
6/28/2025 ... Device deleted
6/28/2025 ... Device configured (null)
Information
Device USB/VIC........... was configured.
Driver Name: null
Class Guid: {00000000-0000-0000-000000000000}
Driver Date:
Driver Version:
Driver Provider:
Driver Section:
Driver Rank: 0x0
Matching Device Id:
Outranked Drivers:
Device Updated: false
Parent Device: USB\ROOT_HUBxx\xxxxxxxx
I have rebootet PC and phone.
Than starting phone in fastboot mode, plugging USB in
Device Manager shows
Android with exclamation mark
I have downloaded the file Qualcomm_USB_Driver_v1.010065.1.zip from qcomdriver.com, extracted it and started Qualcomm USB Driver v1.0.10065.1.exe.
The installation seemed to be successfull.
In the Device Manager is still an exclamation mark, when pluging in the phone while in fastboot-mode.
In Terminal "fastboot devices" brings no results.
When I enter the fastboot mode, for about 0,5s ist written "fastboot mode"
I think, the USB-driver doesn't work.
In the Device Manager:
Other Devices - Android !
An exclamation mark ist shown.
On the webside service.oneplus.com/de/search?key=USB there is not driver for OnePlus Nord N10 5G.
I have tested the driver from developer.android.com/studio/run/win-usb -> usb_driver_r13-windows.zip, but it doesn't work.
The file commandlinetools-win-13114758_latest.zip from the URL "https://developer.android.com/studio?hl=en#download" doesn't work, because it is the wrong file.
It seems, that the right URL is "https://developer.android.com/tools/releases/platform-tools?hl=de" and the file "platform-tools-latest-windows.zip".
When I unpack this file, go to the folder platform-tools, open the terminal and type
adb version
I get the response:
Android Debug Bridge version 1.0.32.
phone: system - developer mode - OEM unlock enabled
Starting the phone in the fastboot (volume up + power) and after that pluging to the PC and write the command
fastboot devices -> no response
fastboot oem unlock -> < waiting for device >
On the linked page "https://community.oneplus.com/thread/64487" is written:
"5. To confirm that it is indeed working, open a command prompt window and enter the following commands:
cd Desktop/android-sdk-windows/platform-toolsadb versionIf it displays "Android Debug Bridge version x.x.xx" it is working. If it is gives an error saying that adb is not a recognized command, it has not been successful. Carefully repeat the steps above if this is the case. Close the command prompt window when you are done."
But the path "platform-toosdb" isn't included in the file commandlinetools-win-13114758_latest.zip.
Extracting the ZIP-file delivers the folder cmdline-tools. I have placed this folder on the Desktop, opened the folder and started the terminal.
PS Directory: C:\Users[username]\Desktop\cmdline-tools> dir
bin
lib
NOTICE.txt
source.properties
PS Directory: C:\Users[username]\Desktop\cmdline-tools> cd platform-toolsadb
cd : The path "C:\Users[username]\Desktop\cmdline-tools\platform-toolsadb" cannot be found
I guess, commandlinetools-win-13114758_latest.zip is the wrong file.