went to the that file you said but I found no reference of that PPA and with the find tool it had no matches. There is nothing with the word ppa or launchpad or lightdm there. What should I do? Maybe make a reference to that ppa and comment it out? I don't know... Also, would it be useful if you had the file or its text?
@fabriono72 You may have a different error message than the original user... Even if slightly
Open a new Thread in Support - if you can capture the error in detail..