I was trying to drag some messages from one folder to another, but I think my mouse button momentarily disengaged and instead I ended up dragging the entire Inbox folder down into one of the subfolders. Once in this state it is impossible to drag the inbox back to where it should go (underneath the resource). When I closed KMail and reopened it, the inbox was back to where it should be, but it had to resync all items in the folder. If moving the inbox isn't sensible or cannot work then it seems to me it ought to be prevented from being done in the first place. Reproducible: Didn't try I'm using disconnected IMAP.
This bug has never been confirmed for a KDE PIM version that is based on KDE Frameworks (5.x). Those versions differ significantly from the old 4.x series. Therefore, I plan to close it in around two or three months. In the meantime, it is set to WAITINGFORINFO to give reporters the oportunity to check if it is still valid. As soon as someone confirms it for a recent version (at least 5.1, ideally even more recent), I'll gladly reopen it. Please understand that we lack the manpower to triage bugs reported for versions almost two years beyond their end of life.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.1 aka 15.12, preferably more recent), please open a new one unless it already exists. Thank you for all your input.