SEE EDIT BELOW
Shaman, that didn't help unfortunately.
Meanwhile I renamed the existing zero-k-portable folder, downloaded a fresh semi-portable zip from itch.io, unpacked, still have the same error.
Regarding the zk-menu.sdz from
Shaman, the strange thing is that
diff zero-k-portable/games/e79f0f3972e0a0644010f0a5b9b81fe7.sdz zkmenu-stable.sdz
Binary files zero-k-portable/games/e79f0f3972e0a0644010f0a5b9b81fe7.sdz and zkmenu-stable.sdz differ
Files have same bytesize but different md5.
But when I unpacked both .sdz files and compared the folders, diff didn't find any difference.
Anyway I moved zkmenu-stable.sdz to games, but still have the same error.
Weird...
EDIT:
Wait, that's not the same error actually, it's just similar so I thought it's the same. The error now is
Zero-K.exe Information: 0 : http://repos.springrts.com/zk/packages/bce544c746dfd4844cd2be613e87ae43.sdp Completed - 222.34k
Zero-K.exe Error: 0 : zk:stable download failed - unexpected end of stream