vSAN Host fails to transition into maintenance mode with an error - Service Unavailable

Products

• VMware vSAN
• VMware vSAN 7.x
• VMware vSAN 8.x

Issue/Introduction

vCenter UI reports 'A general system error occured. HTTP error response: Service Unavailable' when tying to place a vSAN host into maintenance mode.

Hostd.log: 
2025-09-20T17:41:44.432Z info hostd[21009545] [Originator@#### sub=Hostsvc.VmkVprobSource] VmkVprobSource::Post event: (vim.event.EventEx) {
-->    key = 198,
-->    chainId = 1958660544,
-->    createdTime = "1970-01-01T00:00:00Z",
-->    userName = "",
-->    host = (vim.event.HostEventArgument) {
-->       name = "######",
-->       host = 'vim.HostSystem:ha-host'
-->    },
-->    eventTypeId = "esx.audit.maintenancemode.failed",
-->    objectId = "ha-host",
-->    objectType = "vim.HostSystem",
--> }
2025-09-20T17:41:44.433Z info hostd[21009545] [Originator@#### sub=Vimsvc.ha-eventmgr] Event 1996 : The host has failed entering maintenance mode.

Issue persists, when tried from command line:
[root@#####:/var/log] esxcli system maintenanceMode set --enable true -m ensureObjectAccessibility
A general system error occurred: HTTP error response: Service Unavailable

Cause

vsanmgmtd service is not functioning: 
[root@#####:/vmfs/volumes/63fff12e-e1c7a918-e612-############/log] /etc/init.d/vsanmgmtd status
vsanperfsvc is not running

The vsanmgmtd service (also known as the vSAN Health Service) is responsible for vSAN health checks and reporting various component statuses to vCenter. vCenter relies on this service to get accurate information about the vSAN cluster's health. 
If vsanmgmtd fails, vCenter may display incorrect information, and fundamental management operations, including entering maintenance mode, will stop responding or fail. 

Resolution

Stop and start usually fixes the vsanmgmtd service: 
/etc/init.d/vsanmgmtd stop
/etc/init.d/vsanmgmtd start