Comment 8 for bug 1871543

It is the raw clone call that fails:
101 if (__glibc_unlikely (ARCH_CLONE (&start_thread, STACK_VARIABLES_ARGS,
102 »···»···»···»··· clone_flags, pd, &pd->tid, tp, &pd->tid)
103 »···»···»···== -1))
104 return errno;

That returns 11

Limits are the same in good & bad case
$ sudo prlimit --pid 1839368
RESOURCE DESCRIPTION SOFT HARD UNITS
AS address space limit unlimited unlimited bytes
CORE max core file size 0 unlimited bytes
CPU CPU time unlimited unlimited seconds
DATA max data size unlimited unlimited bytes
FSIZE max file size 0 0 bytes
LOCKS max number of file locks held unlimited unlimited locks
MEMLOCK max locked-in-memory address space 0 0 bytes
MSGQUEUE max bytes in POSIX mqueues 0 0 bytes
NICE max nice prio allowed to raise 0 0
NOFILE max number of open files 50 50 files
NPROC max number of processes 3 3 processes
RSS max resident set size unlimited unlimited bytes
RTPRIO max real-time priority 0 0
RTTIME timeout for real-time tasks 200000 200000 microsecs
SIGPENDING max number of pending signals 127615 127615 signals
STACK max stack size 8388608 unlimited bytes