Comment 6 for bug 1817055

Well, I've found out why charm is not complaining: systemctl is marking as SUCCESS despite failures on journalctl.

Also, running an strace on this, I see:
close(3) = 0
stat("/etc/swift/proxy-server.conf", {st_mode=S_IFREG|0644, st_size=2797, ...}) = 0
openat(AT_FDCWD, "/var/run/swift", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = -1 ENOENT (No such file or directory)
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
write(1, "Starting proxy-server...(/etc/sw"..., 55Starting proxy-server...(/etc/swift/proxy-server.conf)
) = 55
pipe([3, 4]) = 0
fcntl(3, F_GETFD) = 0
fcntl(3, F_SETFD, FD_CLOEXEC) = 0
fcntl(4, F_GETFD) = 0
fcntl(4, F_SETFD, FD_CLOEXEC) = 0
pipe([5, 6]) = 0
fcntl(5, F_GETFD) = 0
fcntl(5, F_SETFD, FD_CLOEXEC) = 0
fcntl(6, F_GETFD) = 0
fcntl(6, F_SETFD, FD_CLOEXEC) = 0
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f3707c16a10) = -1 EAGAIN (Resource temporarily unavailable)

So, my guess is that control nodes are overloaded with containers and swift cannot spawn any more workers.
I moved swift-proxy to swift-storage nodes and it worked.