I have the same problem and see in other threads you need the 'strace plymouth --ping', so here it is on my machine:

execve("/bin/plymouth", ["plymouth", "--ping"], [/* 63 vars */]) = 0
brk(NULL)                               = 0x55ee0eea8000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
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=87326, ...}) = 0
mmap(NULL, 87326, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7efc5de10000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libply.so.4", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0000E\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=88632, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7efc5de0e000
mmap(NULL, 2185728, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7efc5d9e9000
mprotect(0x7efc5d9fe000, 2093056, PROT_NONE) = 0
mmap(0x7efc5dbfd000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x14000) = 0x7efc5dbfd000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
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\340\22\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1960656, ...}) = 0
mmap(NULL, 4061792, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7efc5d609000
mprotect(0x7efc5d7df000, 2097152, PROT_NONE) = 0
mmap(0x7efc5d9df000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1d6000) = 0x7efc5d9df000
mmap(0x7efc5d9e5000, 14944, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7efc5d9e5000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/librt.so.1", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\"\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=31744, ...}) = 0
mmap(NULL, 2128864, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7efc5d401000
mprotect(0x7efc5d408000, 2093056, PROT_NONE) = 0
mmap(0x7efc5d607000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x6000) = 0x7efc5d607000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\220\16\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=14632, ...}) = 0
mmap(NULL, 2109712, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7efc5d1fd000
mprotect(0x7efc5d200000, 2093056, PROT_NONE) = 0
mmap(0x7efc5d3ff000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2000) = 0x7efc5d3ff000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\360a\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=144776, ...}) = 0
mmap(NULL, 2221160, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7efc5cfde000
mprotect(0x7efc5cff8000, 2093056, PROT_NONE) = 0
mmap(0x7efc5d1f7000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x19000) = 0x7efc5d1f7000
mmap(0x7efc5d1f9000, 13416, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7efc5d1f9000
close(3)                                = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7efc5de0c000
arch_prctl(ARCH_SET_FS, 0x7efc5de0d040) = 0
mprotect(0x7efc5d9df000, 16384, PROT_READ) = 0
mprotect(0x7efc5d1f7000, 4096, PROT_READ) = 0
mprotect(0x7efc5d3ff000, 4096, PROT_READ) = 0
mprotect(0x7efc5d607000, 4096, PROT_READ) = 0
mprotect(0x7efc5dbfd000, 4096, PROT_READ) = 0
mprotect(0x55ee0d992000, 4096, PROT_READ) = 0
mprotect(0x7efc5de26000, 4096, PROT_READ) = 0
munmap(0x7efc5de10000, 87326)           = 0
set_tid_address(0x7efc5de0d310)         = 24058
set_robust_list(0x7efc5de0d320, 24)     = 0
rt_sigaction(SIGRTMIN, {sa_handler=0x7efc5cfe3c70, sa_mask=[], sa_flags=SA_RESTORER|SA_SIGINFO, sa_restorer=0x7efc5cff1150}, NULL, 8) = 0
rt_sigaction(SIGRT_1, {sa_handler=0x7efc5cfe3d00, sa_mask=[], sa_flags=SA_RESTORER|SA_RESTART|SA_SIGINFO, sa_restorer=0x7efc5cff1150}, NULL, 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [RTMIN RT_1], NULL, 8) = 0
prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0
rt_sigaction(SIGPIPE, {sa_handler=SIG_IGN, sa_mask=[PIPE], sa_flags=SA_RESTORER|SA_RESTART, sa_restorer=0x7efc5d640140}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
brk(NULL)                               = 0x55ee0eea8000
brk(0x55ee0eec9000)                     = 0x55ee0eec9000
epoll_create1(EPOLL_CLOEXEC)            = 3
pipe2([4, 5], O_CLOEXEC)                = 0
epoll_ctl(3, EPOLL_CTL_ADD, 4, {EPOLLERR|EPOLLHUP, {u32=250250048, u64=94480940827456}}) = 0
epoll_ctl(3, EPOLL_CTL_MOD, 4, {EPOLLIN|EPOLLERR|EPOLLHUP, {u32=250250048, u64=94480940827456}}) = 0
openat(AT_FDCWD, "/proc/cmdline", O_RDONLY) = 6
read(6, "BOOT_IMAGE=/boot/vmlinuz-4.13.0-"..., 4096) = 119
close(6)                                = 0
socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0) = 6
setsockopt(6, SOL_SOCKET, SO_PASSCRED, [1], 4) = 0
connect(6, {sa_family=AF_UNIX, sun_path=@"/org/freedesktop/plymouthd"}, 29) = 0
epoll_ctl(3, EPOLL_CTL_ADD, 6, {EPOLLERR|EPOLLHUP, {u32=250265360, u64=94480940842768}}) = 0
epoll_ctl(3, EPOLL_CTL_MOD, 6, {EPOLLERR|EPOLLHUP, {u32=250265360, u64=94480940842768}}) = 0
epoll_ctl(3, EPOLL_CTL_MOD, 6, {EPOLLOUT|EPOLLERR|EPOLLHUP, {u32=250265360, u64=94480940842768}}) = 0
epoll_pwait(3, [{EPOLLOUT, {u32=250265360, u64=94480940842768}}], 64, -1, NULL, 8) = 1
sendto(6, "P\0", 2, MSG_NOSIGNAL, NULL, 0) = 2
epoll_ctl(3, EPOLL_CTL_MOD, 6, {EPOLLIN|EPOLLOUT|EPOLLERR|EPOLLHUP, {u32=250265360, u64=94480940842768}}) = 0
epoll_ctl(3, EPOLL_CTL_MOD, 6, {EPOLLIN|EPOLLERR|EPOLLHUP, {u32=250265360, u64=94480940842768}}) = 0
epoll_pwait(3,