Won't start. Failed to register: timeout reached

## What Happened

Appcenter is failing to start. If you run `io.elementary.appcenter` from the terminal, it hangs for a while before returning a single line of output:
```
Failed to register: O tempo limite foi alcançado
```
It's some sort of "timeout reached" error.

This began after I installed Firefox and ffmpeg-full runtime from Flathub, via appcenter sideload. Both of these flatpaks are running ok, without issues.

Potential duplicate: #1557

## Steps to Reproduce

1. Make sure your system is up to date
2. Sideload Firefox and the ffmpeg-full runtime from Flathub
3. Run Firefox at least once, then reboot the system.
4. Once you're back at the desktop, attempt to run appcenter by clicking its launcher icon or running it from the terminal.

### Logs

```
$ G_MESSAGES_DEBUG=all io.elementary.appcenter
(process:20154): GLib-GIO-DEBUG: 13:24:34.441: _g_io_module_get_default: Found default implementation dconf (DConfSettingsBackend) for ?gsettings-backend?
(process:20154): dconf-DEBUG: 13:24:34.441: watch_fast: "/io/elementary/appcenter/settings/" (establishing: 0, active: 0)
(process:20154): GLib-DEBUG: 13:24:34.441: unsetenv() is not thread-safe and should not be used after threads are created
** (process:20154): DEBUG: 13:24:34.442: ScreenshotCache.vala:34: screenshot path is at /home/dsilva/.cache/io.elementary.appcenter/screenshots
(process:20154): dconf-DEBUG: 13:24:34.443: watch_established: "/io/elementary/appcenter/settings/" (establishing: 1)
(process:20154): GLib-GIO-DEBUG: 13:24:34.444: _g_io_module_get_default: Found default implementation gvfs (GDaemonVfs) for ‘gio-vfs’
Failed to register: O tempo limite foi alcançado
(io.elementary.appcenter:20154): dconf-DEBUG: 13:24:59.470: sync
** (io.elementary.appcenter:20154): DEBUG: 13:24:59.470: ScreenshotCache.vala:71: Screenshot folder size is 933,9 kB
```

## Platform Information

elementary OS 6 Odin
io.elementary.appcenter version: _I have tried --version, `flatpak list` and searched with apt, but I couldn't find this information_
x86_64 arch
Dell Inspiron 15 3583
Intel graphics

## Workaround
After killing it, it works again :)
```sh
killall io.elementary.appcenter
```

And this workaround has a permanent effect. I can no longer reproduce the problem.