Created attachment 112137 [details]
screenshot with ghost column
In kmail 5.8.0 (not available in the version selector, sorry) I got issues with the columns (name, unread, total, size) in the folder list.
Not only that settings aren't remembered (e.g. the size column sometimes gets added when I restart kmail), but also a "ghost" column with no content is added, which leads to a horizontal scrollbar.
See the screenshot for an example.
Kmail is from 18.04, as is akonadi, Qt is 5.10.1, gentoo x64
I can confirm this with kmail 5.7.3 on Debian unstable, Qt 5.10.1, kde frameworks 5.44.
it's a qt5.10.1 bug.
It was fixed in 5.11 but 5.10 is frozen.
Ask to your distro to downgrade qt to qt5.10