Since you can add new users, the trouble is with your user account, not KDE. If this were my system, I would try removing the ~/.kde directory under my user directory, then restart X.
If that does not solve the issue, back up any files you do not want to lose in your user directory. Log out of your user account in X. Switch to a terminal window (CTRL+ALT+F2 will do it). Log in as root. Delete your user account (userdel -r <login>). This will remove the user account, primary group and directory as well as any files or sub-directories.
Add the user account you just deleted (useradd -c <"User's full name"> -m <login>). The users full name must be in double quotes("") as shown. This command will also add an associated primary group for the new user. Give the account a password (paswd <login>). Enter the desired password when prompted.
Now switch back to the X terminal (CTRL+ALT+F7). Log in with the user name and password just created. If you use KDE, the first login will take some extra time to create the KDE specific files, directories and default configuration for the newly created user. You can now customize your new user account as you wish, and restore any unique files you backed up etc.
All items in <> are place holders. For example, <login> should be replaced with the user name you wish to use, and <"User's full name"> should be replaced with the real name of the person who will own the account. If the person is named "George Smith", then that is what you shoule enter. Note that the -c option to useradd is actually a coment, so you could enter anything you want, or not use the option at all. It is customary to use the real name of the user in the account comment to accomodate multi user configurations.