On groovy here and I can't update https://releases.ubuntu.com/20.04/ubuntu-20.04.1-live-server-amd64.iso.zsync using zsync. Had to change it to: http://releases.ubuntu.com/20.04/ubuntu-20.04.1-live-server-amd64.iso.zsync.

strace zsync https://releases.ubuntu.com/20.04/ubuntu-20.04.1-live-server-amd64.iso.zsync
execve("/usr/bin/zsync", ["zsync", "https://releases.ubuntu.com/20.0"...], 0x7fff19f68278 /* 73 vars */) = 0
brk(NULL)                               = 0x564ac4377000
arch_prctl(0x3001 /* ARCH_??? */, 0x7fff829439d0) = -1 EINVAL (Invalid argument)
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=357046, ...}) = 0
mmap(NULL, 357046, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fb90df4f000
close(3)                                = 0
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\360q\2\0\0\0\0\0"..., 832) = 832
pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
pread64(3, "\4\0\0\0\20\0\0\0\5\0\0\0GNU\0\2\0\0\300\4\0\0\0\3\0\0\0\0\0\0\0", 32, 848) = 32
pread64(3, "\4\0\0\0\24\0\0\0\3\0\0\0GNU\0o\340p\t\274W\346\221p0\320R7\r\213\355"..., 68, 880) = 68
fstat(3, {st_mode=S_IFREG|0755, st_size=2029224, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fb90df4d000
pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
pread64(3, "\4\0\0\0\20\0\0\0\5\0\0\0GNU\0\2\0\0\300\4\0\0\0\3\0\0\0\0\0\0\0", 32, 848) = 32
pread64(3, "\4\0\0\0\24\0\0\0\3\0\0\0GNU\0o\340p\t\274W\346\221p0\320R7\r\213\355"..., 68, 880) = 68
mmap(NULL, 2036952, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fb90dd5b000
mprotect(0x7fb90dd80000, 1847296, PROT_NONE) = 0
mmap(0x7fb90dd80000, 1540096, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x25000) = 0x7fb90dd80000
mmap(0x7fb90def8000, 303104, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x19d000) = 0x7fb90def8000
mmap(0x7fb90df43000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1e7000) = 0x7fb90df43000
mmap(0x7fb90df49000, 13528, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fb90df49000
close(3)                                = 0
arch_prctl(ARCH_SET_FS, 0x7fb90df4e580) = 0
mprotect(0x7fb90df43000, 12288, PROT_READ) = 0
mprotect(0x564ac40c2000, 4096, PROT_READ) = 0
mprotect(0x7fb90dfd4000, 4096, PROT_READ) = 0
munmap(0x7fb90df4f000, 357046)          = 0
getpid()                                = 169401
brk(NULL)                               = 0x564ac4377000
brk(0x564ac4398000)                     = 0x564ac4398000
openat(AT_FDCWD, "https://releases.ubuntu.com/20.04/ubuntu-20.04.1-live-server-amd64.iso.zsync", O_RDONLY) = -1 ENOENT (No such file or directory)
write(2, "failed on url https://releases.u"..., 91failed on url https://releases.ubuntu.com/20.04/ubuntu-20.04.1-live-server-amd64.iso.zsync
) = 91
write(2, "could not read control file from"..., 114could not read control file from URL https://releases.ubuntu.com/20.04/ubuntu-20.04.1-live-server-amd64.iso.zsync
) = 114
exit_group(3)                           = ?
+++ exited with 3 +++