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
#WaylandEnable=false
Once you do it
Do
sudo do-release-upgrade
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!