This simply happens from time to time (see attached snapshot), I haven't figured out a way to reproduce it successfully. If I kill Kmail and restart it, it would either fail or remain empty. Generally rebooting the machine will fix it and it won't damage the data. I have never tweaked Akonadi/Kmail a lot except changing the backend to SQLite. Could this be the reason? Reproducible: Sometimes Steps to Reproduce: 1. Open Kmail
Created attachment 101294 [details] Kmail snapshot
By the way, I am using Kubuntu 16.04 and the download URL of Kmail is http://archive.ubuntu.com/ubuntu/pool/universe/k/kdepim/kmail_15.12.3-0ubuntu1_amd64.deb
Same problem here with kmail-17.04.1-3.fc26.
Thank you for reporting this issue in KDE software. As it has been a while since this issue was reported, can we please ask you to see if you can reproduce the issue with a recent software version? If you can reproduce the issue, please change the status to "REPORTED" when replying. Thank you!
I'm afraid that I haven't used Kmail for a while now. I'll mark this as resolved but please feel free to reopen if anyone can still reproduce it.