The problem seems to be kernel version 6.1.30; Just writing image file 'Armbian_23.5.1_Nanopineo_bookworm_current_6.1.30_minimal.img' to SD-card gives the following output via serial console:
U-Boot SPL 2022.04-armbian (May 27 2023 - 19:27:28 +0000)
DRAM: 512 MiB
Trying to boot from MMC1
U-Boot 2022.04-armbian (May 27 2023 - 19:27:28 +0000) Allwinner Technology
CPU: Allwinner H3 (SUN8I 1680)
Model: FriendlyARM NanoPi NEO
DRAM: 512 MiB
Core: 33 devices, 12 uclasses, devicetree: separate
WDT: Not starting watchdog@1c20ca0
MMC: mmc@1c0f000: 0, mmc@1c11000: 1
Loading Environment from FAT... Unable to use mmc 0:1...
In: serial
Out: serial
Err: serial
Net: phy interface0
Error: ethernet@1c30000 address not set.
No ethernet found.
starting USB...
Bus usb@1c1a000: USB EHCI 1.00
Bus usb@1c1a400: USB OHCI 1.0
Bus usb@1c1d000: USB EHCI 1.00
Bus usb@1c1d400: USB OHCI 1.0
scanning bus usb@1c1a000 for devices... 1 USB Device(s) found
scanning bus usb@1c1a400 for devices... 1 USB Device(s) found
scanning bus usb@1c1d000 for devices... 1 USB Device(s) found
scanning bus usb@1c1d400 for devices... 1 USB Device(s) found
scanning usb for storage devices... 0 Storage Device(s) found
Autoboot in 1 seconds, press <Space> to stop
switch to partitions #0, OK
mmc0 is current device
Scanning mmc 0:1...
Found U-Boot script /boot/boot.scr
4121 bytes read in 3 ms (1.3 MiB/s)
## Executing script at 43100000
U-boot loaded from SD
Boot script loaded from mmc
182 bytes read in 2 ms (88.9 KiB/s)
10963883 bytes read in 456 ms (22.9 MiB/s)
8477464 bytes read in 354 ms (22.8 MiB/s)
Found mainline kernel configuration
35417 bytes read in 7 ms (4.8 MiB/s)
504 bytes read in 5 ms (97.7 KiB/s)
Applying kernel provided DT overlay sun8i-h3-usbhost1.dtbo
504 bytes read in 6 ms (82 KiB/s)
Applying kernel provided DT overlay sun8i-h3-usbhost2.dtbo
4185 bytes read in 5 ms (817.4 KiB/s)
Applying kernel provided DT fixup script (sun8i-h3-fixup.scr)
## Executing script at 45000000
Kernel image @ 0x42000000 [ 0x000000 - 0x815b18 ]
## Loading init Ramdisk from Legacy Image at 43400000 ...
Image Name: uInitrd
Image Type: ARM Linux RAMDisk Image (gzip compressed)
Data Size: 10963819 Bytes = 10.5 MiB
Load Address: 00000000
Entry Point: 00000000
Verifying Checksum ... OK
## Flattened Device Tree blob at 43000000
Booting using the fdt blob at 0x43000000
EHCI failed to shut down host controller.
Loading Ramdisk to 4958b000, end 49fffb6b ... OK
Loading Device Tree to 4951a000, end 4958afff ... OK
Starting kernel ...
From here on it hangs.
The board runs fine with kernel 5.15.93 and Bookworm userspace from its usual SD-card that I manually upgraded to Bookworm (edit sources lists and added armbian.gpg key). That dist-upgrade did not upgrade the kernel, still kept at 5.15.93. Also I saw no change in U-boot.
As a trial, switched to nightly in armbian-config, that installed kernel 6.1.30, but also same hang as the official image. So I rolled-back that trial, and now the question is, why does this kernel 6.1.30 hang?