I frequently have this problem where, when I log on, the desktop icons are missing. The way to fix it is to go to activities, delete the current or alternative activity, and create a new activity based on "Desktop Icons", then the icons reappear.

I could never figure out what caused it, but since the last time I changed my wallpaper to use a file in my home directory, rather than one of the installed ones, it has started happening again. Could there be a link?

OK, after playing around and logging off and on a few times, it seems my icons are restored each time, but I don't know what I did, and I am still not sure it has anything to do with my custom wallpaper located in my home directory.
It may be that there was no active default activity (is that possible?), as the last time I went to edit them, I pressed the big "play" button on one, and since then, the icons are always re-appearing on login.

My problem is still not resolved

Unfortunately, sometimes the icons are still all missing when logged in, and I have to create a new activity to get them back.

I notice that on the right hand side there is a scrollbar (on the desktop), but scrolling all the way down still does not reveal anything.

Is there maybe some config or config folders I can delete to ensure my KDE config is in pristine condition?