ExtensionError (sphinx.ext.intersphinx._load) : exception: not enough arguments for format string

Describe the bug

When the intersphinx inventory is reached through a SSL-aware HTTP proxy (standard in company networks), there might be a redirect happening.
This redirect seems to trigger a bug in the intersphinx._load code

loading intersphinx inventory 'python' from https://docs.python.org/3/objects.inv ...
intersphinx inventory has moved: https://docs.python.org/3/objects.inv -> https://<private.proxyhost.net>:<port>/<path>?origurl=https%3A%2F%2Fdocs.python.org%2F3%2Fobjects.inv&<other parameters>

Extension error (sphinx.ext.intersphinx._load)!

Versions
========

* Platform:         win32; (Windows-11-10.0.22631-SP0)
* Python version:   3.12.11 (CPython)
* Sphinx version:   8.2.3
* Docutils version: 0.21.2
* Jinja2 version:   3.1.6
* Pygments version: 2.19.2

Last Messages
=============

None.

Loaded Extensions
=================

None.

Traceback
=========

      File "C:\_dev\python_ws\16_vne\python-dq4ts-lib\.nox\doc\Lib\site-packages\sphinx\events.py", line 415, in emit
        raise ExtensionError(
    sphinx.errors.ExtensionError: Handler <function load_mappings at 0x000001A834D14720> for event 'builder-inited' threw an exception (exception: not enough arguments for format string)

How to Reproduce

Simply run sphinx with at least one intersphinx link (for example documenting a class that extends stdlib str, or referencing :class:str in one of the rst documents)

Environment Information

Platform:              win32; (Windows-11-10.0.22631-SP0)
Python version:        3.12.11 (main, Sep 18 2025, 19:45:51) [MSC v.1944 64 bit (AMD64)])
Python implementation: CPython
Sphinx version:        8.2.3
Docutils version:      0.21.2
Jinja2 version:        3.1.6
Pygments version:      2.19.2

Sphinx extensions

`sphinx.ext.intersphinx`

Additional context

No response

Start at the sphinx.ext.intersphinx._load entry point and reproduce the failure with an intersphinx link through an SSL-aware HTTP proxy that redirects the inventory request. Done means the redirected inventory loads without the “not enough arguments for format string” ExtensionError.