You need to manually adjust product id and manufacturer id in the firmware file. Had this on some flashes in the past where it got flipped in the boot block (no idea why) whilst upgrading and serial showed that uboot refused the tftpd flash after download ( the 64% means it got through) because of wrong ids.
You take the openwrt factory bin, edit it with xvi32 and shoot it on the box to repair it. Then you can force sysupgrade to the original downloaded sysupgrade image without modifications and are fine for future updates again. I've somewhere explained it in the past on this forum (pls search).
Made three of my archers live again until today
Pls rename the topic to tftpd flash failure on archer c7 boot loop fix to help others find it.