Hello,

After more bug hunting, the web server at hydra.gnu.org is back up.  You
should now be able to get substitutes from there and, more importantly,
mirror.hydra.gnu.org will be able to do that too.

If you notice any issues related to substitutes, please report them
here!

(In the past 24 hours or so, you may have experienced problems with
substitutes when the web server at hydra.gnu.org was unstable.  Notably,
it could drop connections prematurely, which one could experience even
when connecting to mirror.hydra.gnu.org since it’s just a proxy.)

Thanks,
Ludo’.