The most common problem which seems to arise with kopete in this forum is connection to the yahoo protocol. This thread is dedicated to the best ways of alleviating the connection problem so users don't have to create a new thread everytime.
You must have at least kopete 0.8.4 for yahoo support
You can check your version through the Help menu > About Kopete
Some steps to troubleshoot:
[list]Check your internet connection, including network settings such as firewalls etc.
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[list]Try removing $HOME/.kde/share/apps/kopete (BACKUP FIRST!). The easiest way to do both at the same time is to rename the folder.
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[list]Upgrade your kopete version. Yahoo! has a notorious reputation for changing its protocol so that third party applications (such as kopete) cannot connect to the network. visit the kopete website at
http://kopete.kde.org for packages.[/list:u][/list][/list][/list]