I'm using Slackware 14.2 running XFCE in my computer. I've noticed that my right control key is behaving as a forward slash /
I tried:

• Login as root and tried: KDE/XFCE/FluxBox, and so on, no problem (Right control works as a normal Control Key, as expected)

• Login as my user and tried: KDE/FluxBox/BlackBox: no problem, except when I run XFCE

So, I suspect this is a problem in some XFCE config related to my user, but I don't know what is going on or even what place I can go to fix this problem.

Have you tried a different keyboard? A lot of times, these types of issues are hardware and not software.

No, it's the keyboard of the laptop.
IMHO, I don't think is a hardware issue, since in other users no problem and even in my user, only in XFCE the problem occurs

Maybe running xev in the different DEs/WMs will reveal what's going on differently amongst them.

See man xev for more.

In xev I could see the difference

• XFCE

• Fluxbox

Have you looked into Settings>Keyboard>Layout?

Yes:
- Use System Defaults: False
- Keyboard Model: Generic 105-key (Intl) PC
- Change Layout option: Win Key + Space
- Compose Key: -
- Keyboard Layout: English (US) French (Azerty) Portuguese Brazil

I have also seen this happen occasionally on my Slackware current system that uses xcb to remap the keyboard that runs XFCE; it disappears when I restart the GUI, which appears to be the only way to get rid of it once it appears.

The good news is that I'm pretty sure that it is not an hardware issue.

Searching and reading man pages, I used xmodmap to change the Control_R key (keycode 105) from Slash to Control_R. It works fine. But only for a moment.
After a while my CTRL_R key behaves again as Slash

I suspect that you have a mistake in your xmodmap or config files somewhere, which is a pain to debug. Trying with a fresh user account (not root!) would be something I would try as a sanity check first as you might just have a left-over config that is conflicting.

btw) the way I could trigger the behavior is by remapping my keyboard twice with a different configuration (i noticed it when playing with a mirrored layout). It likely is a bug deep down in how xcb works which is painful. I now know how not to trigger it, which works for me.