I was getting this error too, but I did not know when it started on my mac (maybe a zed update?).
I tried to uninstall zed, compared releases of elixir-ls, etc, but I finally made it work by making the elixir-ls scripts that zed downloads to be executable. Comparing the version between VSCode and zed, I noticed that the elixir-ls release files were not marked as executable:
By running chmod +x ... things started to work again.
