Were they incremental plugin upgrades or did you skip many versions?
I would need to see how you have this set up to see if I can reproduce it, but first, can you please confirm if the redirects still fail if you disable all non-Toolset plugins and switch to a standard theme such as twentyseventeen?
Turned out to be a plugin compatibility issue - "Admin Bar & Dashboard Control".
This one hides the admin bar and controls access to dashboard for non admin users, no idea why it was conflicting.
Anyway, I have fixed this thanks to you pushing me back to do my homework first 🙂
I replaced this plugin with "Remove Dashboard Access" plugin and added this to my functions to hide the admin bar:
// Disable Admin Bar for all users but admin
add_action('after_setup_theme', 'remove_admin_bar');
function remove_admin_bar() {
if (!current_user_can('administrator') && !is_admin()) {
show_admin_bar(false);
}
}