Open the Synth Rack. How many synths are showing as already inserted?
What program are you running which might have a limit on soft synths?

-francesco-
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)

4GB of RAM is not a lot to work with in Windows 10 and Sonar, especially if you are using virtual instruments like Kontakt.
 
If you open Windows task manager when your system is idle, how much available RAM do you have to work with?
 
My Windows 10 uses about 1.9 GB at idle here.  If I launch Sonar with a new empty project, my RAM use goes up to about 2.0 GB.
 
With these virtual instruments inserted, 2x Dimension Pro + 2x Rapture + 2x Kontakt Player + 10x Cakewalk Sound Center, I am using 3.7 GB.  If I was using larger sample sets the memory would probably get used up way faster.
 
If Windows was running out of memory, you should get a Windows popup warning to close some programs.  I believe that Sonar will use as much RAM as it needs, but allocating memory to Sonar is up to Windows.  Windows will attempt to move anything inactive that it can to the swapfile to free up real memory, but there are limits.
 
So I would definitely check your RAM usage with Task Manager.  If you are not running out of memory, then I would suspect it may be a software issue causing your menu anomalies.

57Gregy
...What program are you running which might have a limit on soft synths?

 
You mean like a sample player?  The Cakewalk VIs are greyed out so I can't see how a 3rd party application could create this issue.  (Well, one possible exception would be something like VE Pro, but he's not running anything like that AFAIK).

Something to consider is that you can pick up Sonar Home Studio for $49, or Sonar Artist for $99.  An easy way to get up to date, and either one can host your virtual instruments.
http://www.cakewalk.com/Products/SONAR/Versions#start
 
These are current versions of Sonar and are supported for Windows 10.

Just to clarify.... you refer to using SONAR 8.5 which goes back a few years, but running on Win10.   Were you running SONAR 8.5 on a different system/OS before?   If so, is this a problem that didn't happen before but is new to the Win10 install?

JonD

57Gregy
...What program are you running which might have a limit on soft synths?

 
You mean like a sample player?  The Cakewalk VIs are greyed out so I can't see how a 3rd party application could create this issue.  (Well, one possible exception would be something like VE Pro, but he's not running anything like that AFAIK).

No, like (older) Music Creator which has a limit on the number of synths which can be run at the same time.
But, duh, it's right there in the OP's first sentence, SONAR 8.5, which has no limit.
That speed-reading course I took 30 years ago isn't working.

I actually installed it directly on Windows 10 and I never ran Sonar before.

Does the fact that it's Sonar LE change anything?

Yes, SONAR LE limited the number of soft synths to 8 IIRC.

Mmh, I see...if that's the case, I guess the solution would be to upgrade the software, like abacab suggested, right? Anyway I was looking at the link he posted and there is a box that says "Total Instruments", wich are still 5 for Sonar Home Studio and 12 for Sonar Artist. This means that Sonar Home Studio wouln't actually change anything from this point of view, while Sonar Artist would do?