This was due to Yahoo upgrading their servers. Matt Rogers, the maintainer for Kopete soon received info from Pidgin maintainers on the authentication changes. Yahoo have upgraded its authentication to version 15 from version 13 and this is now resolved. But the KDE 4.2 based distros may well have to wait for a package on this. In the mean time, revert to Pidgin or compile from Kopete SVN.
Howto on installing from SVN
Step 1: Install svn client from a terminal.
sudo apt-get install subversion subversion-tools
Step 2: Install the tools to compile (generate the executables) from a terminal
sudo apt-get install cmake
sudo apt-get install libqt4-dev kdelibs5-dev [36Mb +]
sudo apt-get install libqimageblitz-dev [33Mb+]
Step 3: Follow are instructions from kopete.kde.org in a terminal
svn co -N svn://anonsvn.kde.org/home/kde/trunk/KDE/kdenetwork
svn up kopete cmake
sudo make install
If you get errors below on building, then it may require the other libraries to build the KDE source.
CMake Error at /usr/share/cmake-2.6/Modules/FindPackageHandleStandardArgs.cmake:57 (MESSAGE):
Could NOT find KdepimLibs (missing: KdepimLibs_CONFIG)
Call Stack (most recent call first):
Solution: Read the KDE4 build instructions and at a terminal type following before you resume where you left off in step 3. This will take a good 270Mb+ but you can install any future source from KDE easily.
sudo apt-get install kdepimlibs5-dev
Test Kopete with the Yahoo Messenger account. The package can be removed with the command
sudo make uninstall kopete