You are right. We have a problem with 2104 SD image. In fact, a wrong version has replaced the true 2104 SD image because of a bug in a jenkins’s build scripts. I’m restoring the right SD image from a backup, right now.
[root@matxine ZynthianOS]# scp 2021-05-18-zynthianos-stable-2104.zip root@os.zynthian.org:/var/zynthian/builds
The authenticity of host 'os.zynthian.org (116.202.123.69)' can't be established.
ECDSA key fingerprint is SHA256:xUB57oKLFfUG/zaNTZ4pQEDE8oZj1SbbyIF7dTHwI7I.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'os.zynthian.org,116.202.123.69' (ECDSA) to the list of known hosts.
2021-05-18-zynthianos-stable-2104.zip 39% 2813MB 1.4MB/s 51:52 ETA
Please, wait an hour from the publication of this comment, delete your current copy of the latest stable SD image and download it again.
UPDATE: wait 2 hours 
Thanks!