Thanks Steve for showing that there is another registry branch for Cakewalk that I didn't know about.
 
There's a curious "MenuLayoutsDirty" (=0) entry under Cakewalk Plugin Manager.  I wonder what that does.
 
 
EDIT: And I also discovered an error in my Platinum branch, namely "EffectsChainArtworkFolder" still set to X1's folder, though this doesn't seem to prevent the new Platinum Effect Chains from finding their background graphics.

FWIW, I can see that Sonar is writing apparently correct entries to the registry by selecting a layout in each of the locations where that can be done and then looking at the registry via regedit. Seems fine. The key SynthRackSynths is present but is empty, unsurprisingly.

ChristopherM
The key SynthRackSynths is present but is empty, unsurprisingly.

Can you set it using the other insert synth menus ?
 

Paul P
 There's a curious "MenuLayoutsDirty" (=0) entry under Cakewalk Plugin Manager.  I wonder what that does.
 
EDIT: And I also discovered an error in my Platinum branch, namely "EffectsChainArtworkFolder" still set to X1's folder, though this doesn't seem to prevent the new Platinum Effect Chains from finding their background graphics.

I have not looked at MenuLayoutsDirty. Dirty flags (if that is what it is) are often used to indicate a change in state to some data which has not been committed. For example the * next to the project name is the visual indicator the dirty flag is set on the project and the project differs from the project on disk.
 
WRT EffectsChainArtworkFolder, yes I have noticed that too. Fortunately the data is fairly static. This is one of those things which results in problems when deleting old versions after installing a new one.

Interesting - having found the empty key, I wondered what would happen if I forced the key to point to a known good layout. So I forced it via Regedit, restarted Sonar, and Insert Synth was no longer greyed out and appeared to work as before.
 
Next step was to attempt to select the default layout via Insert Synth in synth rack. That instantly failed, and the greyed-out was back. I looked again at the registry, and the key had been zapped to blank. So I forced it back to a valid layout, and Insert Synth was instantly available again - surprisingly, I didn't even have to restart Sonar.
 
Subsequently, I have switched between various alternative layouts without problem, but as soon as I select the default, the failure returns.
 
So it looks like some kind of buggy handling of the default layout is causing Sonar to blow a fuse.
 
 

I'd mark this solved, except that having to have Regedit constantly open as a companion to Sonar doesn't seem like acceptable workflow to me.
 

ChristopherM
FWIW, I can see that Sonar is writing apparently correct entries to the registry by selecting a layout in each of the locations where that can be done and then looking at the registry via regedit. Seems fine. The key SynthRackSynths is present but is empty, unsurprisingly.

As Paul suggests, you might be able to recover by setting the default value. The value when not empty is a fully qualified layout file name.
 
This makes me wonder if the VST Inventory has something in it which causes "Default All Plug-ins" layout build process to fail. The VST Inventory may be rebuilt with the Reset button in Preferences > File > VST Settings. Before rebuilding the inventory, I would export that portion of the registry. It a Reset fixes the problem, I would send the registry export to Cakewalk.

@Paul P: It is not set by the other menus, because each seems to have its own MRU entry, which makes sens I suppose. That said, I can think of one MRU that Sonar seems to share for all kinds of things annoyingly, which is the reason when you attempt to open a Pro Channel Preset after opening a Synth Preset, you are invited to find the PCP in the folder which holds the given Synth Presets. This one is so annoying that I often set up a plugin from scratch rather than try to find the preset! But that's another story ...
 

While I was posting #17 I see you figured out forcing the layout works. My guess is the VST Inventory. Export the HKEY_CURRENT_USER\SOFTWARE\Cakewalk Music Software\SONAR\Cakewalk VST X64\Inventory to save what it currently looks like before reseting (if you want to help CW figure it out).
 

@steve cook: There doesn't seem to be a specific value for the key that means default, so presumably a blank key is taken as "use default". It may be that my default is corrupt and I notice that my Plug-in Menu Layouts folder contains a file called default.pgl.tmp - can you kindly confirm that this file exists in your setup? There is no other file except for layout files that I have explicitly created, each of which can be opened successfully in Plug-in Manager.