I think you may have done something wrong. I can't say what, but you are the only person with this problem.
A. - Did XAMPP/LAMPP work correctly before you ran '/opt/lampp/lampp security' ???
B. - Did you test it before running the security fix???
C. - Please tell me exactly what happened when you ran 'opt/lampp/lampp security' with sudo or as su (root)
D. - The ONLY 'user' the security 'fix' works for is the default user 'lampp', what do you think the username is that you supplied?
One other question - are you using a Cyrillic keyboard???