Can't create a new thread (errno 35 "Resource temporarily unavailable")

We are running mariadb galera cluster on three nodes(two data nodes, one garb).
We just had a huge outage.
The error we are getting is

ERROR 1135 (HY000): Can't create a new thread (errno 35 "Resource temporarily unavailable"); if you are not out of available memory, you can consult the manual for a possible OS-dependent bug

The error appeared intermittently on both data nodes.

The servers got plenty of memory, so not a memory issue.

I have attached our `my.cnf`