When I try to issue "su -", I get "su: Authentication failure", and I'm 100% sure password I enter is ok.
I think it started to happen after I issued
chmod +s /usr/bin/screen
chmod 755 /usr/bin/screen
which I believe is unrelated to this problem, and,
chmod -s /bin/su (-s by mistake)
chmod 755 /bin/su
which most probably made the whole mess...
this is not the part of the problem I believe but here's some background why I did that... when trying to make possible for screen sessions to be started automatically on boot under non-root account, I entered something like "su - username -c "/usr/bin/screen -dmS screenname ./executable-file"" in bootmisc.sh, but I was getting "must run suid root for multiuser support", so I tried to fix it, and now I can't login to root account no way
Is there any way to heal the situation?