In reply to @dandellion:dodsorf.as
this sometimes happens, a settings>clearing cache (in element not in android) usually resolves it

unpacking sources
unpacking source archive /nix/store/yzgy1mvxhscsj6w8qzj9i84mjhvzjy7k-source
source root is source
patching sources
updateAutotoolsGnuConfigScriptsPhase
configuring
building
yarn run v1.22.19
warning You don't appear to have an internet connection. Try the --offline flag to use the cache for registry queries.
warning Skipping preferred cache folder "/homeless-shelter/.cache/yarn" because it is not writable.
warning Selected the next writable cache folder in the list, will be "/build/.yarn-cache-1000".
$ node scripts/copy-res.js --offline
warning Cannot find a suitable global folder. Tried these: "/usr/local, /homeless-shelter/.yarn"
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Delete sessions
Found entry in i18n file in both singular and pluralised form Delete sessions
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm deleting these sessions by using Single Sign On to prove your identity.
Found entry in i18n file in both singular and pluralised form Click the button below to confirm deleting these sessions.
Found entry in i18n file in both singular and pluralised form Delete sessions
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Found entry in i18n file in both singular and pluralised form Confirm signing out these devices
Done in 6.62s.
yarn run v1.22.19
warning You don't appear to have an internet connection. Try the --offline flag to use the cache for registry queries.
warning Skipping preferred cache folder "/homeless-shelter/.cache/yarn" because it is not writable.
warning Selected the next writable cache folder in the list, will be "/build/.yarn-cache-1000".
$ webpack --progress --bail --mode production --offline
warning Cannot find a suitable global folder. Tried these: "/usr/local, /homeless-shelter/.yarn"

92% chunk asset optimization TerserPluginerror Command failed with signal "SIGKILL"..13/node_modules/css-loader/dist/cjs.js??ref--7-1!/nix/store/2xfr08c9iy696nq2855r3vb126d2ayby-element-web-modules-1.10.13/node_modules/postcss-loader/src/index.js??postcss!/nix/store/2xfr08c9iy696nq2855r3vb126d2ayby-element-web-modules-1.10.13/node_modules/matrix-react-sdk/res/themes/dark/css/dark.scss 0855r3vb126d2ayby-element-web-modules-1.10.13/node_modules/matrix-react-sdk/res/themes/dark/css/dark.scssss[22m
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
error: builder for '/nix/store/c7qf9drzw9wijxx6vhih2ingj81n8sfx-element-web-1.10.13.drv' failed with exit code 1

Seems like the latest Element Web package is broken? This happened on nixos-rebuild switch --upgrade, with

let
elementWebConfig = {
    default_server_config = {
        "m.homeserver" = {
            base_url = "https://matrix.artemislena.eu";
            server_name = "artemislena.eu";
        };
        "m.identity_server".base_url = null;
    };
    disable_guests = true;
    disable_3pid_login = true;
    showLabsSettings = true;
};
in

and somewhere further down below, in the Caddyfile config, root ${pkgs.element-web.override(old: {conf = elementWebConfig;})}…