I tried to update an Omnia Wifi 6 on HBS which had the experimental FW package list already from an earlier time.
I faced the exact same error as Turris OS 7.0.3 is in RC! - #25 by julien113 . In htop, I saw there is a fw_printenv process hogging one core. When I killed the process, the updater was able to finish and flash the new u-boot correctly.