BadStatusLine at RetrieveContent every run

I have a weird issue so bear with me.

For years I've been running a pyvmomi script that connects to a vcenter API and synchronizes objects to an asset mgmt system API.

This script is running for 4 different vcenter environments and 3 of these still work but a 4th has stopped working. Seemingly without any identifiable cause such as recent changes or upgrades in that vcenter environment.

The symptom is that si.RetrieveContent raises the exception httplib.BadStatusLine.

No more info than that.

The troubleshooting I've done leads me to believe that there's nothing wrong with the login or account yet I see this error on the vcenter server.

2018-05-04T16:24:41.350+02:00 error vpxd[20020] [Originator@6876 sub=[SSO] opID=7fe9d9d1] [UserDirectorySso] AcquireToken exception: class SsoClient::InvalidCredentialsException(Authentication failed: Invalid credentials)
2018-05-04T16:24:41.351+02:00 error vpxd[20020] [Originator@6876 sub=User opID=7fe9d9d1] Failed to authenticate user <svcs_vc_siptrack>
2018-05-04T16:24:43.443+02:00 info vpxd[12420] [Originator@6876 sub=vpxLro opID=03912d57-c8ab-4ee6-8358-a0609eea49d2-38213-ngc-1e] [VpxLRO] -- BEGIN task-internal-2700641 -- EventManager -- vim.event.EventManager.queryEventsById -- 523bdeaa-f6e9-1b75-f8b6-9f38e0e25c99(52889030-62de-39b5-dac6-1e21cbf233e9)
2018-05-04T16:24:43.446+02:00 info vpxd[12420] [Originator@6876 sub=vpxLro opID=03912d57-c8ab-4ee6-8358-a0609eea49d2-38213-ngc-1e] [VpxLRO] -- FINISH task-internal-2700641
2018-05-04T16:24:44.357+02:00 info vpxd[20020] [Originator@6876 sub=vpxLro opID=7fe9d9d1] [VpxLRO] -- FINISH task-internal-2700639
2018-05-04T16:24:44.357+02:00 info vpxd[20020] [Originator@6876 sub=Default opID=7fe9d9d1] [VpxLRO] -- ERROR task-internal-2700639 -- SessionManager -- vim.SessionManager.login: vim.fault.InvalidLogin:
--> Result:
--> (vim.fault.InvalidLogin) {
-->    faultCause = (vmodl.MethodFault) null,
-->    msg = ""
--> }
--> Args:
-->
--> Arg userName:
--> "svcs_vc_siptrack"
--> Arg password:
--> (not shown)
-->
--> Arg locale:
-->

If the vcenter server is saying vim.fault.InvalidLogin then why isn't this reflected in the pyvmomi client?

Start by reproducing the failure at si.RetrieveContent and comparing the client exception with the vCenter authentication logs shown in the report. Done means determining why vim.fault.InvalidLogin is surfaced as httplib.BadStatusLine, or documenting the missing diagnostic needed to distinguish the causes.