Version:            (using KDE 3.5.8)
Installed from:    Compiled From Sources
Compiler:          gcc 4.0.3 
OS:                Linux

I had KDE 3.5.5 installed in /opt/kde-3.5.5. It worked fine.
I then installed KDE 4.0.0 in /opt/kde-4.0.0.
I backed up my ~/.kde directory before using KDE 4.
I used KDE 4 then went back to KDE 3
I restored my ~/.kde directory and went back to KDE 3.
Also restored all environment variables back to their KDE values (KDEDIR, KDEDIRS, KDE_PREFIX, PATH).

When I got back to KDE 3, I had several problems.
1) My KMenu was entirely broken. Categories (Games, Graphics, Internet) had the standard 'folder' icon instead of their regular icon. Most of the actual menu entries were missing.
2) kinfocenter shows no modules to view
3) kcontrol shows only one module, "Network", which doesn't actually allow configuring anything. Selecting Network only shows its name in the main area to the right.

I thought that possibly something in my KDE 3 installation had been overwritten. So I installed a fresh KDE 3.5.8 to /opt/kde-3.5.8. I also tried removing ~/.kde and making a new user with no ~/.kde or other config directories in his home directory. Even with a fresh KDE 3 and a fresh user home directory,
the same problems occur.

This is very odd and I have not had problems with KDE 3 like this before.
Normally KDE 3 will always make a default set of menus for a fresh user.

Also, kmenuedit does not show the normal menus either.
One clue might be that kmenuedit displays the following when loading:
kmenuedit: WARNING: Could not read /home/foo/.config/menus/applications-kmenuedit.menu

But I do not know why that would suddenly matter, when KDE 3 menus have always worked before...

kcontrol prints:
kcontrol: WARNING: No K menu group with X-KDE-BaseGroup=settings found ! Defaulting to Settings/

and kinfocenter prints:
kinfocenter: WARNING: No K menu group with X-KDE-BaseGroup=info found ! Defaulting to Settings/Information/

Maybe kcontrol, kinfocenter (and thus KServiceGroup?), and the menu are all looking for a list of items in the wrong place?

Apparently the menus use the ~/.config/menus/applications-kmenuedit.menu and ~/.local/share/applications/ directories, ignoring all KDE's directories and menu information in /opt/kde-3.5.8/share/applnk.
As for kcontrol and kinfocenter, I don't know...

In addition to bug fixes, suggested work-arounds would also be helpful...