Howard, you are a GOD! You saved my life! Many many many thanks.
BTW: I did exactly what has been descibed above! I was changing (especially disabling) some of the STUDENT's rights. I confirmed the changes... and got the error/admin/section errorr (http://docs.moodle.org/en/error/admin/sectionerror). Since I didn't have a clue what to do with it, I backed up and lost all my admin rights.
P.S. One minor thing I've found out right now. After I log out of my moodle profile, a warning shows right above the login text-field saying something like: "Your session has expired, please login again." [I have a Czech mutation so I don't know the exact formulation in English.] Any ideas?