You get the error Unable to fetch item from backend when entering IMAP folder

There are a number of possible reasons for this and it is something the KMail team hopes to tackle in time. Meanwhile, there are some workarounds:

Workaround 1
  • edit ~/.local/share/akonadi/mysql.conf

  • Under the [mysql] section, add: binlog_format=row

If this does not work, try workaround 2 (below).

Workaround 2

This one is mostly a matter of restarting so KMail can fetch those pesky items... Some possible steps:

Use Alt+F2 or Konsole to type: kquitapp kmail, then wait a minute, then akonadictl stop, wait a minute, type akonadictl start, wait a minute, type kmail. This stops KMail (closing all windows), stops the KMail backend, starts the KMail backend, starts KMail. Having a working internet connection increases the chances of success. Sometimes, you can also just do kquitapp kmail, wait a minute, and start KMail again. Often, a few restarts seem to be needed. It is unclear what is the reason for this, but on bad network connections it is more likely to happen.

See also the below item for how akonadiconsole can be helpful.