After upgrading to 2.4.0, I get this error:

    ./propellor --spin myserver
    Building propellor-2.4.0...
    Preprocessing library propellor-2.4.0...
    In-place registering propellor-2.4.0...
    Preprocessing executable 'propellor' for propellor-2.4.0...
    Preprocessing executable 'propellor-config' for propellor-2.4.0...
    Propellor build ... done
    Git commit ... done
    Enter passphrase for /home/user/.ssh/id_rsa: 
    Identity added: /home/user/.ssh/id_rsa (/home/user/.ssh/id_rsa)
    Counting objects: 253, done.
    Delta compression using up to 4 threads.
    Compressing objects: 100% (253/253), done.
    Writing objects: 100% (253/253), 173.59 KiB | 0 bytes/s, done.
    Total 253 (delta 172), reused 0 (delta 0)
    To root@myserver:/var/lib/git/private/propellor.git
       d81fb7d..6f7f041  master -> master
    Push to central git repository ... done
    From myserver:/var/lib/git/private/propellor
       d81fb7d..6f7f041  master     -> origin/master
    Pull from central git repository ... done
    ** warning: git branch origin/master is not signed with a trusted gpg key; refusing to deploy it! (Running with previous configuration instead.)
    Sending privdata (87652 bytes) to myserver ... done
    From .
     * branch            HEAD       -> FETCH_HEAD
    Sending git update to myserver ... done
    Warning: The package list for 'hackage.haskell.org' is 47 days old.
    Run 'cabal update' to get the latest list of available packages.
    Resolving dependencies...
    Configuring propellor-2.4.0...
    cabal: At least the following dependencies are missing:
    exceptions -any
    propellor: failed to make dist/setup-config
    Shared connection to myserver closed.
    propellor: remote propellor failed

As in https://propellor.branchable.com/todo/issue_after_upgrading_shared_library/, manually running "make clean" on the server fixed the issue