Server becomes unresponsive and users cannot login on RHEL if new processes cannot be forked/created

Issue

• Server becomes unresponsive, may show "init: cannot fork" messages. Server can be pinged but login attempts through ssh or local login attempts fail. Some software can log fork failures, for example:
  • Cron register entries similar as this at /var/log/cron: "(...) crond[9478]: (CRON) error (can't fork)"
  • sshd register entries similar as this at /var/log/secure when login attempts are made: "(...) sshd[9233]: error: fork: Resource temporarily unavailable"