after executing sudo apt-get install --reinstall update-manager/dapper everything started to work!
what is the difference between this command line and the reinstall function that I have used via synaptic UI? -- Cannot open Repositories panel in Synaptic Package Manager https://launchpad.net/bugs/50441 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
