Invalid MIT-MAGIC-COOKIE-1 key caused by user nobody

[rasmus91]

OS and Version

elementary OS 5.1 (Hera)

Flatpak version

Flatpak 1.6.2

Description of the problem

Whenever i try to run a flatpak, e.g. flatpak run <app-name> I simply get a message that i have an invalid magic cookie, here's the example with slack

Gtk-Message: 21:30:41.689: Failed to load module "pantheon-filechooser-module"
Invalid MIT-MAGIC-COOKIE-1 key
(slack:2): Gtk-WARNING **: 21:30:41.690: cannot open display: :99.0
[fake-sandbox: zypak-sandbox] No data could be read (host died?)
[fake-sandbox: zypak-sandbox] Quitting Zygote... 

Steps to reproduce

I've no idea why or how it does this. This time, It happened when I had booted my laptop at home.
I clicked the icon, but i just got the spinning cursor. So then i tried running from terminal and I got the above message.

[rasmus91]

Right! So I tried a few things in the terminal, then went to do reboot bot it told me that user nobody was logged into the system...

So i went to good ol' FF and searched for that. Turns out nobody is sometimes used by systemd (or its units?) to run certain services, then i recalled i installed ofono-phonsim-autostart today, and while starring at the terminal i noticed it creating a systemd file. So I tried apt removing that package, rebooting and behold; flatpaks work on my system again.

I don't know if its the specific service that mucks it up, or the nobody user in general, can someone else try with the ofono-phonesim-autostart package, and then maybe tell if there's another package that uses user nobody, that i can test with, so we can ascertain where exactly the problem lies? Because this seems to somehow have messed with the X session

[TingPing]

Maybe the service is overwriting the XAUTHORITY env var or file.

[rexkerr]

Had the same problem with being unable to launch flatpak apps that used X, with a similar MIT-MAGIC-COOKIE-1 error. Searching brought me here, and sure enough I'd installed the ofono-phonesim-autorestart package today, and removing it fixed the problem. (For me, it was on Ubuntu 18.04 -- some packages would start [e.g. Audacity], and others would not [e.g. Evolution, Pinta])