Admin Page Error 503 after upgrade to1.8.8

Provider: Z.com

Virtualization Type: KVM

System: Centos 7.5

Installation option: Upgrade from old installation

OLS or Enterprise?
OLS

Installing from official server or mirror server?
official

Replace JS/CSS files to JS Delivr?
yes

Install Memcached extension for PHP?
yes

Install LiteSpeed Memcached?
yes

Install Memcached?
no

Install Redis extension for PHP?
yes

Install Redis?
yes

Problem:

After upgrade to 1.8.8. I got 503 errors when access Cyber Panel Admin page

Just restart your VPS/machine and wait.
Try after 5mins.