Unable to enter nix-shell

[asheshambasta]

Thanks a lot for the tutorials; I think this is an incredible resource.
However, when trying to follow the getting set-up instructions at https://qfpl.io/posts/reflex/basics/exercises/introduction/index.html, after having set up the binary-caches (I'm on 20.03.2459.c9d124e39db (Markhor)); I see the following error:

...
building '/nix/store/a4zpzqwsjfdlr9a1g6948p00j8lrg563-cabal2nix-haskell-src-exts-util-0.2.5.drv'...
installing
building '/nix/store/qg2k0w7wf94vxkyrc68gr9fpj4wzb3c8-source.drv'...

trying https://bitbucket.org/wuzzeb/webdriver-utils/get/a8b15525a1cceb0ddc47cfd4d7ab5a29fdbe3127.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (22) The requested URL returned error: 404
error: cannot download source from any mirror
builder for '/nix/store/qg2k0w7wf94vxkyrc68gr9fpj4wzb3c8-source.drv' failed with exit code 1
cannot build derivation '/nix/store/7fhw8c1n39kpsdpm4bxsfd7hf2353fb9-cabal2nix-hspec-webdriver.drv': 1 dependencies couldn't be built
error: build of '/nix/store/7fhw8c1n39kpsdpm4bxsfd7hf2353fb9-cabal2nix-hspec-webdriver.drv' failed
(use '--show-trace' to show detailed location information)

---

I've not had the chance to debug this further, but I thought it'd be worth reporting.