Authentification failure for PostgreSQL server (9.3)
I found a temporary solution to my problem. I edited the pg_hba.conf of the 9.3 server to say trust in the first two active lines. And after restarting (sudo service postgresql restart) I can connect to the server again using pgadmin. The downside is that the server might now not be password protected (I will open a separate question if I encounter any problems with that).

    # Database administrative login by Unix domain socket
    local   all             postgres                                trust

    # TYPE  DATABASE        USER            ADDRESS                 METHOD

    # "local" is for Unix domain socket connections only
    local   all             all                                     trust
    # IPv4 local connections:
    host    all             all             127.0.0.1/32            md5
    # IPv6 local connections:
    host    all             all             ::1/128                 md5
    # Allow replication connections from localhost, by a user with the
    # replication privilege.
    #local   replication     postgres                                peer
    #host    replication     postgres        127.0.0.1/32            md5
    #host    replication     postgres        ::1/128                 md5