This problem need to be solved on server configuration. Please check yourself or ask your host support about

• mod_security disabled

• <IfModule mod_substitute.c>
  SubstituteMaxLineLength 10M </IfModule>
  

• IF you use PHP Fast CGI set  values
  
  pm.max_children = 75
  pm.process_idle_timeout = 10s
  pm.max_requests = 500

• IF you use Litespeed set  values
  LSAPI_CHILDREN = 75 
  PHP_LSAPI_MAX_REQUESTS = 500
  
  or

LSPHP_Workers 100

ref. https://www.litespeedtech.com/support/wiki/doku.php/litespeed_wiki:php:concurrency#reached_max_children_process_limit

If above not helps ask server provider support to check error logs