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

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

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