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)
From URL https://community.oneplus.com/thread/1541327 I downloaded Qualcomm_Drivers_Setup.exe from URL https://drive.google.com/file/d/1zKPFtcc2X_Nf70mcvn9TBu60bHl6Q3cP/view
and installed on Windows 11 Pro.
Options:
o WWAN-DHCP is not used to getIPAddress
o ETHERNET-DHCP is used to getIPAddress (choice)
Phone hold Volume Up and Volume Down, plug in USB-cable, release Volume Up and Volume Down
*** Nothing happens ***
According to https://community.oneplus.com/thread/1541327
Method to get into EDL-mode
(Unlock the developer options by tapping the build number 7 times rapidly in About Phone/About Device.)
(Open the Developer options and enable USB debugging.)
Connect phone to PC and type on Windows 11 Pro:
cmd [Enter]
adb devices [enter]
A pop-up appears up on my phone: USB-Debugging -> "always allow"
adb reboot edl [enter)
screen ist black
Device-Manager: COM & LPT: Qualkomm HS-USB QDLoader 9008 (COM3)
MsmDownloadTool Start was still acitvated and MSMDownloadTool automatically worked for 6 seconds, now showing:
COM: COM3
Status of Connection: N/A
Status of Communication: Multi-immage identify by target sw id is disabled
Progress of Download: (green bar)
Time: 6 s
Status of Last Communication: Device not match image (20886)
phone first start only with Vol.UP+Vol.Down+Start -> choose Start -> Start-Button
*** Still Android Version Oxygen OS 11..4.BEB86AA
Does "Device not match image" mean, that the MsmDownloadTool-Version is not matching to the OnePlus Nord N10 5G?
I had downloaded https://onepluscommunityserver.com/list/Unbrick_Tools/OnePlus_Nord_N100/EU_BE83BA/Q/OnePlus_Nord_N100_EU_OxygenOS_10.5.2.zip from https://devices.ubuntu-touch.io/device/billie2/