Currently, the installation image (lede-lantiq-xrx200-BTHOMEHUBV5A-installimage.bin) is booted.
In Step 5, backup of the stock firmware, the nanddump file is much smaller than the expected 128 MiB.
Repeating the nanddump or restarting the whole installation did not yield different results.
root@lede:/# nanddump --file ~/hh5a.nanddump /dev/mtd4
ECC failed: 0
ECC corrected: 0
Number of bad blocks: 1
Number of bbt blocks: 4
Block size 131072, page size 2048, OOB size 64
Dumping data starting at 0x00000000 and ending at 0x08000000...
root@lede:/# ls -l ~
-rw-r--r-- 1 root root 55320576 Nov 5 21:41 hh5a.nanddump
I have transferred the nanddump to my PC.
I guess continuing with the installation is not safe?
Connecting the flash drive before power up and leaving it connected solved the problem.
Waiting for the front LEDs to turn blue before starting nanddump was also necessary. (As mentioned in the Installation Guide PDF)