When two skype instances are started at the same time (which can happen in linux), the autologin will automatically be disabled. If no email address is set and the password is only stored within skype it will be impossible to login to skype again.
The solution is, to have a backup of the ~/.Skype directory. There are two entries which need to be adjusted:
<credentials> section from the config.xml in the backup directory to ~/.Skype/user/config.xml<SavePassword> in ~/.Skype/shared.xml to 1