I'm with a new Windows 10 installation and I've installed Kontakt 6. 
But all my older project are in Kontakt 5.
Now, when I try to open these project, I get this:
 

 
However, Kontakt 6 is completely compatible with Kontakt 5 (also it has the same name "Kontakt.dll"). 
How can I solve this?

While I do not use Kontakt, I am thinking that the Kontakt 6 overwrote the Kontakt 5 DLLs.  Perhaps because the Kontakt 6 CLSIDs are different to the Kontakt 5 CLSIDs , CbB is not finding the CLSIDs for Kontakt 5.

What I would do is reinstall Kontakt 5.  Then reinstall Kontakt 6, but this time, see if you can do a custom install so you can specify a particular folder (can be a subfolder of your normal VST folder) for the Kontakt 6 DLLs. 

After the reset and scan, I know you will two copies of Kontakt (5 and 6), but at least you will be able to open projects that use Kontakt 5 while being able to select Kontakt 6 for new projects.

It might also be worth dropping a note to Native Instruments to see if they can offer some insight.

My apologies if I am way off.  My Kontakt knowledge is very little.  Hopefully some who uses Kontakt 5 and 6 successfully will come along and clarify this (or even correct me).

Kontakt v6 and v5 do not have the CLSID's and Kontakt 6 does not automatically replace Kontakt 5, apparently you removed Kontakt v5 and you shouldn't have.
Two different versions of Kontakt (v5 and v6 in this case) will exist side by side.
Also, Kontakt 6 is kontakt.dll. Kontakt 5 is kontakt 5.dll
 

I keep all versions of Kontakt on my machine, so If I open an older project that I used Kontakt 2, it will load those samples/patches. 

I think there's some confusion about Kontakt's backwards-compatibility....
 
In a nutshell, it only means Kontakt can import libraries designed for older versions of Kontakt.  IOW, Kontakt 6 has no problem loading your 7-year-old Indiginus library that has a basic Kontakt 2 requirement.
 
It does not mean you can load a DAW project that originally used, say, Kontakt 4, with Kontakt 6 and have it intelligently load the track with all its parameters intact.  For that, you need to have Kontakt 4 (or whatever version you used) installed.

gustabo
Kontakt v6 and v5 do not have the CLSID's and Kontakt 6 does not automatically replace Kontakt 5, apparently you removed Kontakt v5 and you shouldn't have.
Two different versions of Kontakt (v5 and v6 in this case) will exist side by side.
Also, Kontakt 6 is kontakt.dll. Kontakt 5 is kontakt 5.dll
 

Is there any way to "cheat" CbB so that the current Kontakt 6 has the same internal number as the previous Kontakt 5?

rogeriodec
Is there any way to "cheat" CbB so that the current Kontakt 6 has the same internal number as the previous Kontakt 5?

Roger, see my post above.  Your track expects Kontakt 5, so you just need to reinstall it (K5 and K6 can coexist side by side).
 
If it were me, I'd uninstall K6 first, install K5, then reinstall K6.