Upgrade from 17.04 to 17.10 stuck because of errors

[PJJ]

My upgrade from 17.04 is stuck because of 'errors' and a locked session.
Fix is ctrl + alt+ F2, then
login, then
"loginctl unlock-session,"
then ctrl+alt+F7.

This supposed to continue the update. Well, in my case it does not.

How can one fix this problem, please?
It is all about Plasma and the suggestion is to use another theme. But how can I without logging in?
Only 'plasma' shows under themes.

I can get as far as the login screen and log in. It gets stuck there with a list of error messages. All related to Plasma. Too hard to read. There obviously several packages missing or did nto extracted.

Computer says it cannot start or go further with Plasma as theme. Suggests switching themes. But the theme box only allows Plassma.
Nvdia driver installed and working under 17.04.

How can I switch themes?

[PJJ]

Come on, guys. Nobody knows anything about this and how to fix it?
I do not believe that.

What is the solution to the issue, please?

[dequire]

Hello. Sounds like there are some missing or broken packages somewhere. First step in the process would be to get to a terminal: At login press and hold <ctrl> + <alt> + F2. This will get you to a terminal login. After login run

sudo apt update && sudo apt full-upgrade

and look for errors. If errors are found, you can run

sudo dpkg --configure -a

Check the results and repeat as necessary until it continues to fail or fixes the issue(s) and report back here as to the results. Good luck.

[PJJ]

I realized that something was wrong with one of the 'missing' files. Instead of trying o fix the problem, I opted for a clean install (preserving existing data). I purchased a DVD with 17.10 at ebay and installed the os.
Smooth install, and, as a bonus, I avoided bricking my laptop by installing Kubuntu 17.10.