Hi all.
I was composing with Sonar 8.5 and everything went more and less fine, until at a certain point, without any manual changes, I couldn't insert virtual synths anymore. More specifically, the boxes that appear after "Virtual Synths" under "Insert" menu (Cakewalk Sound Center, Rapture, Vst Plugins ecc.), became grey and I couldn't do anything with them while until the previous track i could use them to insert VSTs without any problems. At first I thought it was related to an hypotetical maximum number of midi tracks, but deleting tracks doesn't fix it. The only thing I could do was to undo my last action ("insert midi track" or "insert virtual synth") only to fall back into the same problem when I did it again. Since I need various tracks on a single project to work on my compositions I am getting quite mad at this, because it means that from a certain point on, I can't do anything and it just seem that I have limited actions regardless the number of tracks. It doesn't even always happen after 5 tracks because previously it happened when I only had 3. Please, someone help me!

Somebody, please?

Just to be clear, are you saying this happens with a brand new project?  When did this behavior start? 
 
There are many ways to insert a VSTi.  What happens say, when you try from the Synth Rack or right-click on a track?

It started from my very first project. When I insert my first midi track with insert ---> virtual synths ---> Cakewalk Sound Center (for example) everything works fine. I can repeat this, creating other midi tracks up to a certain numerb of times (5 or even 3). After that I can't repeat this process and create a new midi track with vsts anymore. Anyway I tried to figure out what is the cause and it is the effect (regardless it is Cakewalk Sound Center or Kontakt or whatever) because before inserting a new vst track everything is fine, while after doing that I can't insert one anymore and that is the only thing I did in between (if I undo inserting my last midi track everything is fine again while if I redo there it comes again).

Seems odd that after 3 or 5 tracks, the boxes become greyed out.  Sounds like the behavior of a demo version.
 
Did you install from a disc or a download?  If the latter, was it from somewhere other than the Cakewalk site? 

I installed it from the disc and registered it regularly on Cakewalk.

I've also never heard of this. Could it be memory problems? How much RAM does your system have?

Yes, it would be helpful if you gave a brief rundown of your system specs. And are you running 32 or 64 bit Sonar?
 
Also, a long shot -- I remember right around the time of Sonar 8/8.5 that "Run as administrator" became a thing to watch out for.  Try starting Sonar that way (Right-click on shortcut, choose Run as administrator) and see if there's any change.

I also think that the version of Windows used could be relevant.  There are bundled C++ program libraries and API's that older programs such as Sonar 8.5 were developed around, that may not be compatible with the current Windows versions.  I also have Sonar 8.5, but have not attempted to use it since Windows XP.
 
I would download a demo version of the latest Sonar and compare it with what you have on your system.

I tried running as administrator but it doesn't chage anything.
 
I was beginning to think about a RAM problem too. Is there any way to give more RAM to Sonar? Anyway I' m using Windows 10 and 64 bit version and here are more info about my pc:
 
Processor: Intel(R) Celeron(R) CPU N2840 2,16GHz
Installed Ram: 4.00 GB (3,89 usable GB)
 
(Or at least that's what I found on my control panel)