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.
