I use my own custom icons all over my 2 vertical KDE panels. Recently I migrated a user directory from another machine with minimal problems but there are 2 stubborn icons that resist. The hostname on the receiving system is not the same as on the sourced one.

The App-Launcher icon (icon only, it still works) has vanished and while I can replace it via the Edit-Applications dialog and command the system to save it doesn't show up on the panel. If I try to put a new App-Launcher wiget on the panel it does not "take" i.e. double clicking it does NOT place it on the panel, not even if I first delete the iconless one. Hint the Enable-launch-feedback, Place-in-systray, and Only-show-in-kde checkboxes remain ghosted tyhroughtout

Same problem with a Quick-Access widget, another peculiarity is that both of these exist(ED) on both vertical panels. Smells like a permissions problem but I cannot nail it. I don't want to rebuld two complete panels if I can avoid it. I reset perms to very liberal on the entire user directory to no avail.

probably a consequence of your migration.
this sounds potentially harmful. don't do that.

Everything is backed up and it's only experimental, but thanks for the caution :-)

it's not only caution.
it's also the kind of warning that, if ignored, causes people to post threads about resulting problems.
in other words: you borked it, you fix it.

Actually it would seem that the problem has no more to do with migration then with my migrating habits, less than nothing to be precise, or am I a harsh judge of character.

When the left vertical panel is set to a width of 33 pixels everything is in place, just like on the right panel. Increasing the width of either to 34 causes the top two icons to vanish but in such a way that the place they had occupied remains empty and continues to act exactly as if the icons were still there. Icons similarly vanish off the bottom also but the places they occupied do not remain and continue the function.

There are a number of problems looking for KDE solutions in the case of vertical panel placement but that is not the topic so I won't go into it.