If you are using VMware’s vCenter to manage your virtualization environment, the chances that you encounter this famous 503 Service Unavailable message are pretty high.
503 Service Unavailable (Failed to connect to endpoint: [N7Vmacore4Http20NamedPipeServiceSpecE:0x00007fd8c00155b0] _serverNamespace = / action = Allow _pipeName =/var/run/vmware/vpxd-webserver-pipe)

Most of the time, a restart would get you going again. But today, a simple restart won’t cut it. I had to try a few different ways to finally get the login page again.
First of all, if SSH isn’t an option, you will need to get access to the console, which mostly you can through the ESXi server management portal.
Press Alt+F1 to get to the login screen and use root user to log in. I hope you still remember your root password. If not, check this one out. Once logged in, launch shell, by
shell.set --enabled true shell
Then, run the following to check the services status.
service-control --status --all
If you see any services that stopped, run the following to start all services.
service-control --start --all

Then wait for another 15 minutes before checking the web portal again.
While you are in the console, you should also check and make sure none of the logical drives should be full.
df -h
A few other resources to look if above doesn’t help.
- Reindex the database – https://www.running-system.com/vmware-vcsa-error-503-service-unavailable/
- VMware KB – https://kb.vmware.com/s/article/67818
- Due to vSphere web client – https://chrisdsibug26.wordpress.com/503-service-unavailable-error-when-connecting-to-the-vcenter-server-using-the-vsphere-web-client/