Just updated tonight with Discover, rebooted and now i cannot enter my desktop.

I can change terminal (CTRL-ATL-F1...) and login with my user but i'm unsure of where to look at.
Under /var/log i don't see specific kde log ?

For now, i'm using my Linux Mint 19 backup os....

Boot Neon system in recovery mode, login with root.
write command: systemctl default
Restart system

I just ran into this same problem upgrading my mom's PC from Kubuntu 16.04 to 18.04. Turns out the desktop for login got set to "Cairo Dock" for some reason. I clicked on the text near the bottom of the login screen (for desktop session) and chose "Plasma" instead, and then everything worked fine.

systemctl default in rescue mode gives some error about swap, disk, etc... and never come back to the prompt (need to force shutdown).

I can select Plasma or Plasma (Wayland), under Plasma i can enter my password but pressing enter do nothing more than freezing the screen (cannot click on anything on login screen) but i can CTRL-ALT-F1 to go into a terminal and login from there... Also tryed systemctl default from there... no error but did not help...

When trying to log with Plasma (Wayland), i got black screen with small squared color lines in middle-bottom of the screen (like problem with the video driver).

At least i can run on Linux Mint for now but if i need to re-install, i think i'll get back with Manjaro-KDE... Never thought that stable base (Ubuntu 16.04) with latest KDE could break like this

OK MY FAULT!
Just fixed : Almost no space on my drive caused this symptom...
Delete VM files to free up spaces, now 22GB free on 40GB... Now i'm posting on my Neon install...

It would be good to have some warning instead of not letting me in...

I'm experiencing the same issue, tried like 15 times to login but no luck.
Compared to my last reboot, I installed VSCode, nothing else.
Just logged in with Ubuntu desktop while trying not to cry, and everything works fine.

I don't know if it's related, but after typing my password, I saw my Terminator window (as it's starting after login, so does Rambox), then switch back to a KDE black loading screen for 5 seconds, then I'm headed back to the desktop and everything is frozen. This happened each and every time tonight.

I'm gonna try to uninstall Rambox - have no knowledge neither time to try to disable it only after login for KDE - and will let you know.

But still, may it be from an broken program on login or KDE itself, this is a blocker for me. I'm writing this from the Ubuntu desktop, and if it's not for KDE I couldn't switch from MacOS. KDE Plasma meant my first time really ditching MacOS / Windows for an DM that was better than all combined. The lack of user feedback ("this program is stuck", even a BSOD with a message) is really problematic.

The lack of professionnal confidence is also an issue. Not all the time, but I was in the train and needed to write a very important email to a client.

Let's be clear, you definitely made - in my opinion - the best DM, whatever the OS. MacOS is really weak compared to what KDE can offer. I'm just a bit tough because you're close, so freakin close to beat them all in any field, that's I hardly handle disappointment. But this is 5% on a 95% total satisfaction (spent 3 months fully happy, except for cookies being dropped to often).

Keep up with the amazing work, and thanks so much for making this great DM open, available for all.
Cheers,