Syncing config of kde can be a pain.
Especially akonadi is not very robust in case of missing
config files.

So if your kaddressbook is suddenly empty, you might try:

vpoinot, your error message looks like the one I found on my system.
Maybe you should give this a try: Just copy the main config file from
(/usr/share/config/akonadi/mysql-global.conf)
/etc/akonadi/mysql-global.conf
to the appropriate directory of your user
$HOME/.config/akonadi/mysql-local.conf
and uncomment the line which reads
sql_mode=strict_trans_tables


From: https://bbs.archlinux.org/viewtopic.php?pid=695037


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Not to be confusing with 100% CPU usage of virtuoso-t.
This leads to non-reactions of kmail.
(Best: restart kde, restart of akonadi does not help)


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No autocompletion/suggestion:

- Removing and readding systemsettings -> personal information did not help