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