cargo upgrade returns 401 consistently even after changing registry token

% cargo upgrade                            
    Checking virtual workspace's dependencies
Error: status code '401 Unauthorized': the request was not authorized

cargo upgrade --verbose dont seem to give any extra details on the error.

Tried the following:

1. removing all cache
2. renewing the token in config.toml
3. cargo search --registry internal-registry-name internal-feature works fine with the token enabled
4. other cargo commands like cargo clean, cargo build, cargo test all work fine

This was working fine for many months and seeing this issue only the last couple months now.

Start by reproducing cargo upgrade with the internal registry and the token in config.toml, then compare its authentication behavior with cargo search --registry internal-registry-name and the working build and test commands. Trace the cargo upgrade entry point until the source of the 401 is identified; done means dependency upgrades work with the configured registry token and the failure is no longer reproducible.