I completely agree with Christopher.
 
I'm not convinced my registry is in the best of health at any time, so whilst it still works, it ain't getting diddled with!

.reg files are plain text you can visibly check everything it's doing in notepad.  But I also understand not wanting to take any risk to verify this.  The information is here, Cakewalk is welcome to use it.  But I won't be reporting anything to their support, they burned that bridge on this very issue.
 

ChristopherM
I have found only two ways of reliably blowing up Synth Rack: selecting "default" layout in Browser Synth tab (i.e. Synth Rack): or selecting "default" via (main menu) Insert - Soft Synth. Oddly, in the latter case, the main menu is happy to be set to "default" (i.e. it works fine and can later be reset to a specific layout) but the collateral damage is that Synth Rack is then crippled until I go to regedit and fix the blank key.

Your problem probably has something to do with the default layout itself, as I'm sure you've concluded.  In my experiments, I only used layouts that I had previously created, so only fixed layouts that don't undergo any kind of change.  I never selected the "Default All Plugins" of which I have two that always appear.  Well, I did at the beginning until I realized that it wasn't obvious what their purpose or behaviour was.  One of them gives me VST2 and VST3 categories, the other only VST2, but there's only one default.pgl.tmp.  The default layout appears to be continuously rebuilt upon opening Sonar, as is shown by the default.pgl.tmp's modification date.
 
I spent no time looking into the behaviour of this default layout processing.
 

I'm wondering whether I have one particular plug-in that somehow prevents Sonar from creating the default on the fly and that Sonar's code is not robust enough to take such a thing in its stride ... but wondering is as far as I am going to take it, because I just don't have the time to spend finding the rogue by any process of elimination, especially since there may be more than one rogue.

I am now getting this problem in the latest version of SONAR (22.4.0 Build 27 [2016.04] - x64) where the "Insert Synth" button in the Synth Rack Browser is "greyed out" and cannot be activated (i.e., the "Insert Synth" button after you click the “+” button in the Synth tab of the Browser).  Did you guys ever figure out the cause and resolution to this problem.  I have used this method for inserting VIs for many years and have never seen this problem before now.

I regret to say that I have not found any way to fix this other than to go into the registry via regedit and fix the corrupt keys. The fix lasts only for so long before Sonar corrupts the keys again, so this is not a practical solution. The Problem Report CWBRN-45498 that I made at the beginning of February still says -

Status Information

The current status of your report is
New
This is a new problem report. It has not yet been reviewed by Cakewalk.
 
This makes me wonder what Cakewalk actually does with Problem Reports if after nearly 4 months no-one has even looked at it. None of the other methods for inserting a soft synth is particularly workable either. The newish Add Track thing looked promising but its bizarre way of ordering the list (without the ability to use Layouts) makes it unattractive, too.
 
I don't feel well supported by the Bakers here, I'm sorry to say.

It's not a specific entry, it's the total number of entries.  And yes Cakewalk has totally dropped the ball on this.  I won't submit any more bug reports to them because of the nonsense responses this bug has gotten from their front line support.
 

I wonder why the keys are getting corrupted in your cases? I have never seen this behavior and it makes me wonder if there is an antivirus that is intercepting Sonar's write to the registry and corrupting it?
 
Regards,
Dan

The only AV that I use is Windows Defender and I have no experience of that messing up anything. I don't think that anything is intercepting Sonar's writes - I think Sonar is screwing it up all by itself. As I said above, I wouldn't be surprised if this is to do with one-or-more rogue plugs, but unless Sonar says what's happening, there is no practical way to find out.
 

Windows Defender probably isn't the cause...so I agree with your point. It is strange that it isn't more common. I have well over 600 plugins and use a custom layout.
 
Regards,
Dan