vCenter 503 Service Unavailable | How to Deal with It

Have you ever got the vCenter 503 Service Unavailable error while browsing vCenter Server or vCenter Server Appliance (VCSA)? There are many causes that could lead to this issue. This article will share with you the recommended troubleshoot steps.


By Crystal / Updated on November 21, 2022

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Case: vCenter 503 Server Unavailable (Failed to connect to endpoint)


Hello guys, I was encountered this error on my host machine when accessing the vcenter server with vSphere Client (HTML5) - partial functionality, any solution or recommendation to solve my current issues on my vCenter Server 6.5 thank you guys and GOD BLESS US ALL...

- Question from

Are you experiencing the same problem when trying to browse to vCenter Server or VCSA? The famous “vCenter 503 Server Unavailable (Failed to connect to endpoint)” error often happens when you logging in or accessing the vCenter or VCSA using your vSphere Web Client.

vCenter 503 Service Unavailable

You may need to troubleshoot the causes to resolve this error. This article covers the possible causes of the error “vCenter 503 Server Unavailable” and shares several ways to fix it.

Possible causes of the “vCenter 503 Server Unavailable” error

Before troubleshooting, you should understand what does the error 503 Service Unavailable vCenter implying.

According to VMware, a “503 Service Unavailable” error is an HTTP response status code that indicates that the server is temporarily unable to handle your request. Therefore, the 503 error is a server-side error, it means that the problem exists within the vCenter Server or website (server) you are trying to access, and not your computer (client).

There are a number of causes that may lead to this error, for example:

  • The vCenter Server currently down because of maintenance.
  • The reverse Proxy service on vCenter Server is down.
  • The vSphere web client service is down.
  • Mis-configured Firewall settings on a Windows installation of vCenter Server.

Based on these possible causes of this error, VMware provides a series of troubleshooting steps for 503 Service Unavailable vCenter 6.5. The next part will show you these steps.

How to troubleshoot the error “vCenter 503 service unavailable”

Before you start, if the vCenter Server is just started, please remember to wait a few more minutes or start it on a different client, and then retry.

This is because the vCenter Server may be busy with other tasks and not able to acknowledge your request, and some needed services may haven’t started yet. If you still get the same error, you can refer to the following steps to troubleshoot.

1. Confirm the status of the VMware vCenter Services.

Ensure all required services are up, running and functional for both the vCenter Server device and any external Platform Services Controller (PSC) device also.

For example, you can open the Appliance Management Interface in a web browser by entering the IP address or the FQDN (a Fully Qualified Domain Name) for vCenter and port 5480, and then check the status of vCenter services in the Services section. if the vSphere Web Client service is Started, and the vCenter Server service is Stopped, the VMware 503 Service Unavailable error can occur.

Check vCenter services

2. Check if there are disk space issues on the vCenter Server or the vCenter Server Appliance.

The disk space issue does not directly cause a 503 Error Response Code, but the services down due to a failure causes the error.

You can run the following command to check if the partitions have enough free space on VCSA:

df -h

VCSA disk space

3. Check whether the vCenter certificate expired.

vCenter CLI

VMware provides a 503 Service Unavailable vCenter flow chart for a step-by-step troubleshooting process:

VMware troubleshoot process

Effective data protection: batch backup VMs managed by vCenter

To avoid losing the password to your accounts, you may have it recorded. And to avoid losing data on VMs managed by vCenter, you will also need to make backups just in case. However, vCenter Server does not provide the feature of virtual machine backup. And that makes backup software a commonly accepted choice.

Here I introduce you to a VMware backup software AOMEI Cyber Backup, it enables you to backup multiple VMs either managed by vCenter Server, or on a standalone ESXi host.

Agentless Backup: create complete and independent image-level backup for VMware ESXi and Hyper-V VMs.
Multiple VM Backup: batch backup large numbers of VMs managed by vCenter Server, or multiple VMs on a standalone ESXi host.
Multiple Storage Destinations: backup to a local drive, or network destinations like NAS.
Automated Execution: create backup schedules to automate backups daily, weekly, or monthly.
Restore Entire VM: restore instant available VMs from any selected restore points to an original or new location.

AOMEI Cyber Backup supports VMware ESXi 6.0 and later versions. Next, I will show you how to batch backup VMs managed by vCenter Server with AOMEI Cyber Backup in 4 simple steps. You can click the following button to download the 30-day free trial.

Download Free TrialVMware ESXi & Hyper-V
Secure Download

4 Steps to batch backup VMs managed by vCenter Server

1. Bind Devices: Access to AOMEI Cyber Backup web client, navigate to Source Device > VMware > + Add VMware Device to Add vCenter or Standalone ESXi host. And then click > Bind Device.

Add VMware ESXi host

2. Create Backup Task: Navigate to Backup Task > + Create New Task, and select VMware ESXi Backup as the Backup Type.

vmware esxi backup

3. Set Task Name, Device, Target, Schedule, and Cleanup as needed.

  • Task Name: you can change the task name or use the default name with an ordinal.
  • Device: batch select large numbers of VMs managed by vCenter Server for centralized backup.

Select virtual machines vCenter

  • Target: select to back up to a local path, or to a network path.
  • Schedule (optional): perform full, differential, or incremental backup, and automate execution according to the frequency you specified.

Backup schedule type

  • Cleanup (optional): automatically delete the old backup copies that exceed the retention period you specified.

backup cleanup

4. Run Backup: Click Start Backup and select Add the schedule and start backup now, or Add the schedule only.

Start Backup

Created backup tasks will be listed and monitored separately, for restoring, progress checking and schedule changing.

When restoring, you can also restore to new location to create a new VM in the same or another datastore/host directly from the backup, saving the trouble of re-configuring the new VM.

Restore to new location


The vCenter 503 Service Unavailable error is an HTTP response status code that indicates that the server is temporarily unable to handle your request. The problem exists within the vCenter Server and there are many possible causes for it. This article shows you how to troubleshoot this issue.

In addition, this issue is usually temporary. If the above steps still cannot solve your problem, you can also backup your important data and try update vCenter.

Crystal · Editor
Crystal is an editor from AOMEI Technology. She mainly writes articles about virtual machine. She is a positive young lady likes to share articles with peolpe. Off work she loves travelling and cooking which is wonderful for life.