Since I have enabled the ca-* related feature I think I have encountered a bug.
I filed it here
https://github.com/NixOS/nix/issues/4903
I think that when those ca-* flags are on sometimes nix tries to fetch unexisting cache path.
I’ve tried to enable -vvv to see what exactly was happening.
I’m waiting to test without the flags to see If the bug persists.
Unfortunately the bug itself is quite transient. It sometimes appears repeatedly, but I can’t isolate a minimum set of features to trigger it.