The target user has too many processes or threads.
alone without each process limit Linux threads, just on the system process of the total limit
(thread is essentially on Linux has Shared address space of the process),
to check the number of threads:
the cat/proc/sys/kernel/threads – Max
to increase the number of threads:
echo 100000 > /proc/sys/kernel/threads-max
There is also a limit to the number of processes (and threads) that a single user can create, ulimit/getrlimit details about these limits.