GVim Windows: `:Copilot setup` fails with E-32603: Request signIn failed

Hello there,

I am on Windows, using gVim 9.2. I followed the installation instructions in README.md to install the copilot plugin. When running `:Copilot setup` within vim, I get the following error message:

```
Copilot:E-32603: Request signIn failed with message: Response content-type is missing (status=200)
```

The only point where I deviated from the readme instructions during the installation is that I cloned the plugin repo at a different location

```
%USERPROFILE%/vimfiles/pack/plugins/start/copilot.vim
```

instead of

```
%USERPROFILE%/vimfiles/pack/github/start/copilot.vim
```

What do you think I could be doing wrong?