cannot update, with medibuntu.sos-sts.com sources.

Thank you for taking the time to report this bug and trying to help make Ubuntu better. However, it seems that you are not using a software package provided by the official Ubuntu repositories. Because of this the Ubuntu project can not support or fix your particular bug. Please report this bug to the provider of the software package. Thanks!
If you are interested in learning more about software repositories and Ubuntu the following pages should be informative:
1. http://www.ubuntu.com/community/ubuntustory/components - information about Ubuntu repositories
2. https://help.ubuntu.com/community/Repositories - information regarding managing repositories.

You might want to check your sources.list to make sure there are not some references to outdated sources.list.

Thank you for taking the time to report this issue and helping to make Ubuntu better. Examining the information you have given us, this does not appear to be a bug report so we are closing it and converting it to a question in the support tracker. We appreciate the difficulties you are facing, but it would make more sense to raise problems you are having in the support tracker at https://answers.launchpad.net/ubuntu if you are uncertain if they are bugs. For help on reporting bugs, see https://help.ubuntu.com/community/ReportingBugs#When%20not%20to%20file%20a%20bug.