At least stable works:
➜ ./scripts/librem5-flash-image --stable
2023-10-01 19:40:34,303 INFO Looking for librem5r4 luks byzantium image
2023-10-01 19:40:34,433 INFO Found disk image Build "stable" 'Last stable librem5r4 build' from Fri Jun 23 22:21:40 2023
2023-10-01 19:40:34,856 ERROR handle_other(): Mutual authentication failed
2023-10-01 19:40:35,910 INFO Found uboot Build 85 from Thu Aug 25 15:22:41 2022
2023-10-01 19:40:35,910 INFO Downloading to ./tmp_librem5-flash-image_62domuyz
2023-10-01 19:40:36,047 INFO Downloading image from https://storage.puri.sm/librem5/images/byzantium/latest/librem5r4/luks/artifact/librem5r4.img.xz
Download: 0%| | 5357568/4149978124 [00:03<51:14, 1347890.61it/s]
Decompr.: 1%|▍ | 35545118/4500000256 [00:03<12:49, 5800987.97it/s]