Slack can't open browser on Fedora 35

[Ian Pilcher]

First time Flatpak user here, trying to use the Slack on Fedora 35 (now
that they're retired the RPM).

I am able to run the app, and the Slack window appears.

> $ flatpak run com.slack.Slack
> [2 preload-host-spawn-strategy] Running: /app/bin/zypak-helper child - /app/extra/lib/slack/slack --type=zygote --enable-crashpad
> Initializing local storage instance
> (electron) Sending uncompressed crash reports is deprecated and will be removed in a future version of Electron. Set { compress: true } to opt-in to the new behavior. Crash reports will be uploaded gzipped, which most crash reporting servers support.
> [2:0428/155535.430906:ERROR:bus.cc(397)] Failed to connect to the bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
> [2:0428/155535.430968:ERROR:bus.cc(397)] Failed to connect to the bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
> LaunchProcess: failed to execvp:
> xdg-settings
> [2:0428/155535.508868:ERROR:bus.cc(397)] Failed to connect to the bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
> [2:0428/155535.508905:ERROR:bus.cc(397)] Failed to connect to the bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
> [2:0428/155535.508925:ERROR:bus.cc(397)] Failed to connect to the bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
> [50:0428/155535.514775:ERROR:sandbox_linux.cc(377)] InitializeSandbox() called with multiple threads in process gpu-process.
> [2:0428/155535.526788:ERROR:browser_main_loop.cc(267)] Gtk: gtk_widget_add_accelerator: assertion 'GTK_IS_ACCEL_GROUP (accel_group)' failed
> Gkr-Message: 15:56:05.513: secret service operation failed: org.freedesktop.DBus.Error.ServiceUnknown

When I click on the "Sign In to Slack" button, however, it never brings
up a browser window in which I can log in.

> Failed to call portal: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name is not activatable

As I mentioned, this is my first time attempting to use Flatpak, so I
could really use some advice on how to troubleshoot this issue.

Thanks!

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