Hi, I solved the problem, even though I don't know why it happened.
I tried by removing the user and its home from the system, re-create the user and start everything from scratch, and this time it worked. I think something went wrong and maybe there was a conflict with previous configuration files of some sort (I tried to build from sources before using kdesrc build script), maybe that could be the culprit. Thank you!

Hi, sorry to bump this post, but I found the cause (and the solution) of the problem; since removing the entire kde-devel's home doesn't work if development packages of Qt5 are not removed as well (and it is frustrating to rebuild everything from scratch), I think it might be useful to someone that incurs in the same problem.
As pointed out in the previous posts, the problem is that some Qt5 libraries from the system-wide installation of Qt5 are dynamically linked in the kdeinit4 executable. This is not caused by the compiling/installation of kdelibs itself, but by the compiling/installation of attica.
Attica is now built by default against Qt5, hence kdeinit4 executable dynamically links to the new attica built against Qt5 and fails to launch. You can see this by issuing "ldd libattica.so.1.0".

The solution is to add this cmake option

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cmake-options -DQT4_BUILD=TRUE

in your kdesrc-build configuration file, in the module-set related to attica. Then, delete the old attica libs in $KDEDIR/lib or $KDEDIR/lib64 and rebuild the module attica and kdelibs. In this way kdeinit4 is no more linked to Qt5 libraries.
However, there should be a warning somewhere about this behaviour, in fact with a default run of the script without enforcing the QT4_BUILD option, one can end with a KDE environment that doesn't start. Now that most systems have both Qt installed, it's a bit tricky to find that the culprit is the attica built by the script, at least for me.
Hope it helps anyone with the same problem

This should be reported as a bug against Attica and kdelibs, as the build systems shouldn't allow Qt 5 and Qt 4 libraries to be used together in the same binary - as you've found they are completely incompatible.

Should it be reported as a bug against attica or against kdesrc-build, since the solution is simply to add the option QT4_BUILD in the configuration file of the script?

Against kdelibs - it shouldn't be trying to use an incompatible Attica.

Ok, thank you, I will file the bug as soon as I have time!