If after modifying a few flies, Lubuntu does not allow you to connect, there are a few things you might want to try.
You have modified your configuration and when you login, you can not go through.
There are manly two files which you might want to modify:
You may want to amend these files. For me, the simplest way is to renamme these files (e.g. to .OLD) then to reconnect: Lubuntu will recreate them correctly and you will be able to login.
You only have to compare the .OLD files to the existing ones and to correct the problems, to get your previous configuration back.
Hoping that will help you to get through Login !