'UserVars.PXEBootEnabled' is invalid or exceeds the maximum number of characters permitted." (vSphere 6.0)

While trying to scan a host (ESXi 6.0.0 Update 3 build-5050593) I got a rather cryptic failure message in vCenter (Version 6.0.0 Build 3617395):

Scan entity <name of ESXi host> VMware vSphere Update Manager had an unknown error. 

Check the events and log files for details.

VSPHERE.LOCAL\Administrator

Looking in vpxd.log I found this:

2017-07-31T14:02:18.402-04:00 info vpxd[04232] [Originator@6876 sub=Default opID=6c569171] [VpxLRO] -- ERROR task-internal-499427 -- EsxHostAdvSettings-217 -- vim.option.OptionManager.queryView: vim.fault.InvalidName:

--> Result:

--> (vim.fault.InvalidName) {

-->    faultCause = (vmodl.MethodFault) null,

-->    name = "UserVars.PXEBootEnabled",

-->    entity = <unset>,

-->    msg = "Received SOAP response fault from [<cs p:000000000fca7ae0, TCP:vapelhhost01.local:443>]: queryView

--> Received SOAP response fault from [<cs p:1f3178a0, TCP:localhost:8307>]: queryView

--> 'UserVars.PXEBootEnabled' is invalid or exceeds the maximum number of characters permitted."

--> }

--> Args:

-->

--> Arg name:

--> "UserVars.PXEBootEnabled"

2017-07-31T14:02:18.423-04:00 info vpxd[04240] [Originator@6876 sub=vpxLro opID=4d28aa2d] [VpxLRO] -- BEGIN task-internal-499428 -- host-217 -- vim.HostSystem.GetConfigManager -- 527c060d-9a03-8628-5919-25e69cbd2735(523b7dec-3982-790b-1ff5-4c18aa9cc5ce)

followed shortly with:

2017-07-31T14:02:18.465-04:00 info vpxd[04264] [Originator@6876 sub=Default opID=759a9f26] [VpxLRO] -- ERROR task-internal-499429 -- EsxHostAdvSettings-217 -- vim.option.OptionManager.queryView: vim.fault.InvalidName:

--> Result:

--> (vim.fault.InvalidName) {

-->    faultCause = (vmodl.MethodFault) null,

-->    name = "UserVars.ImageCachedSystem",

-->    entity = <unset>,

-->    msg = "Received SOAP response fault from [<cs p:000000000fca7ae0, TCP:vapelhhost01.local:443>]: queryView

--> Received SOAP response fault from [<cs p:1f3178a0, TCP:localhost:8307>]: queryView

--> 'UserVars.ImageCachedSystem' is invalid or exceeds the maximum number of characters permitted."

--> }

--> Args:

-->

--> Arg name:

--> "UserVars.ImageCachedSystem"

2017-07-31T14:02:18.588-04:00 info vpxd[04160] [Originator@6876 sub=vpxLro opID=8f6d0c3] [VpxLRO] -- BEGIN task-internal-499432 -- host-217 -- vim.ManagedEntity.GetName -- 527c060d-9a03-8628-5919-25e69cbd2735(523b7dec-3982-790b-1ff5-4c18aa9cc5ce)

Web searches don't reveal anything useful. I don't have AutoDeploy enabled/configured so PXE shouldn't be an issue. There are no firewalls between the hosts and vCenter/VUM. This is the first time using VUM on this instance of vCenter. A fresh reboot of the host didn't help.

Has anyone else encountered this or know what can be done to fix it?