An unspecified error has occurred: Adding read ACL for uid 1000 to /run/media/my_user

/run/media/my_user isn’t created there for nothing will mount. If you create this directory manually it works as expected:

mkdir -p /run/media/my_user
chown my_user:my_user /run/media/my_user
chmod 750 /run/media/my_user

Here is /etc/polkit-1/rules.d/50-mount-disk.rules

/* -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*- */

// Mount disk by plugdev group rules for polkit

polkit.addRule(function(action, subject) {
    if ("org.freedesktop.udisks2.") == 0 &&
        subject.isInGroup("plugdev")) {
             return polkit.Result.YES;

Have I failed to update a config file or something?
What is the correct solution?

You have a fresh kernel? In the updated kernel this is doing automaticly. tmpfs must have acl support.