Ok, I might get downvoted to oblivion and get my answer privilege taken away because this is only an improvement (Or I would like to think so) but I have to answer this.
Note: this is an improvement over @zanna 's answer in this same thread, but I don't have comment privilege yet so I'm posting this as a separate answer.
If you have followed @zanna 's answer in this thread and you are using SLiM display manager, and you have decided you don't like this very much/would like to go back to gdm3 (No Unity dock and takes away most of the useful functionalities in Ubuntu, you cannot even tap-to-click without a lot of tinkering around) maybe this will help you get back to gdm3.
The 'Started BPFilter' bug happens mostly in Ubuntu 18.04 (and I've also seen a few cases in 18.10). Go on, Install SLiM as @zanna has suggested and check if that works.
Note 2: there are a couple of answers in other thread suggesting you use LightDM. It has not worked for me. I reached the login screen and I was forever re-directed to the same screen even if I input the right credentials. You are free to go play around using the command line from there (Ctrl + Alt + Del or something) but here is a thread on why that wouldn't probably work.
Thread (Even LightDM doesn't work for me, Is there no respite!): [https://www.reddit.com/r/Ubuntu/comments/9hagsd/ubuntu_1804_just_keeps_returning_to_login_screen/]
Note 3: (If you have come this far, I take it you have also gotten rid of your Nvidia Drivers, which for most of you did not exist to begin with! [Yeah, I did the purge too only to find they don't exist]).
Solution: (Not really, since you need to move away from 18.04).
As the top answer has suggested, in /etc/gdm3/custom.conf uncomment the line that says
Once you do it
This should get you the next immediate Ubuntu version. However, this is not the Development version (which actually works with gdm3 for me).
So next, do:
sudo do-release-upgrade -d (-d flag for development branch).
This will ask you to edit a text file and change the version from LTS to normal (Follow the error message!).
Once this is done, go into recovery, and do:
dpkg-reconfigure gdm3 and choose gdm3 this time (Don't choose SLiM/ LightDM).
That is it! Tada! Everything works now!