Here is my /etc/inittab:

id:3:initdefault:

# System initialization (runs when system boots).
si:S:sysinit:/etc/rc.d/rc.S

# Script to run when going single user (runlevel 1).
su:1S:wait:/etc/rc.d/rc.K

# Script to run when going multi user.
rc:2345:wait:/etc/rc.d/rc.M

# What to do at the "Three Finger Salute".
ca::ctrlaltdel:/sbin/shutdown -t5 -r now

# Runlevel 0 halts the system.
l0:0:wait:/etc/rc.d/rc.0

# Runlevel 6 reboots the system.
l6:6:wait:/etc/rc.d/rc.6

# What to do when power fails.
pf::powerfail:/sbin/genpowerfail start

# If power is back, cancel the running shutdown.
pg::powerokwait:/sbin/genpowerfail stop

# These are the standard console login getties in multiuser mode:
c1:1235:respawn:/sbin/agetty 38400 tty1 linux
c2:1235:respawn:/sbin/agetty 38400 tty2 linux
c3:1235:respawn:/sbin/agetty 38400 tty3 linux
c4:1235:respawn:/sbin/agetty 38400 tty4 linux
c5:1235:respawn:/sbin/agetty 38400 tty5 linux
c6:12345:respawn:/sbin/agetty 38400 tty6 linux

# Local serial lines:
#s1:12345:respawn:/sbin/agetty -L ttyS0 9600 vt100
#s2:12345:respawn:/sbin/agetty -L ttyS1 9600 vt100

# Dialup lines:
#d1:12345:respawn:/sbin/agetty -mt60 38400,19200,9600,2400,1200 ttyS0 vt100
#d2:12345:respawn:/sbin/agetty -mt60 38400,19200,9600,2400,1200 ttyS1 vt100

# Runlevel 4 used to be for an X window only system, until we discovered
# that it throws init into a loop that keeps your load avg at least 1 all
# the time. Thus, there is now one getty opened on tty6. Hopefully no one
# will notice. ;^)
# It might not be bad to have one text console anyway, in case something
# happens to X.
x1:4:respawn:/etc/rc.d/rc.4

# End of /etc/inittab

So I followed your suggestion and tried switching users in runlevel 4 through the kde start menu. Alas, it was impossible. Once I start a second X session, when I try to switch back to the first one, I get the blank screen. :(
So that does not work on my system, neither in runlevel 3, nor in 4.

I assume that you tried to switch via the KDE menu and not via CTRL-ALT-Fx?

I must admit I am lost here. From your earlier posts, I understand that you modified your X config from the default. All I can think of at this point is to go through thoses changes and check them.

I had exactly the same problem as you before on a laptop and it was because the keyboard layout was wrong, and had changed from Xorg 6.8 to 6.9. One side effect was that the '@' sign was no longer available either. I still think that your issue might stem from the same cause. Did you google to find the correct config for your keyboard? Do you have another keyboard to test, possible with a different layout?

I asked you about a second computer from which you could connect through SSH, is that an option?

I assume that you tried to switch via the KDE menu and not via CTRL-ALT-Fx?
Yes, I tried to switch via the KDE menu, not via the keyboard. So I do not think the issue is with the keyboard, since in runlevel 4 with the switch user option and working only with the mouse, I still cannot switch back to the other user.

I must admit I am lost here. From your earlier posts, I understand that you modified your X config from the default. All I can think of at this point is to go through thoses changes and check them.
The only changes to xorg.conf I have introduced concern, adding a FontPath, the Modeline and DisplaySize lines in Section Monitor, and the DRI section at the end.

Did you google to find the correct config for your keyboard? Do you have another keyboard to test, possible with a different layout?
No, I did not google for I do not know what to look for. Besides I do not think it is a keybord layout problem, cos it exists in runlevel 4 and working with only the mouse, just pressing the buttons in the kde menu.

I asked you about a second computer from which you could connect through SSH, is that an option?
No I did not try connecting from SSH. Probably I will try these days, but I do not know how this could help me. Probably I should connect to my computer when in "freezes" after attempting to switch sessions? And what should I check for?

Thank you for your attention.
Regards,
Martin