I just upgraded to Kingston Build. After a few days my Start Screen quit showing up and I get an error screen:
 
Sonar Platinum
 
X an unnamed file was not found.
 
Same error when I do File | Start Screen..
 
Looking for a fix
 
 
 

Have you tried reinstalling Sonar?
Edit: Maybe the first step would be to roll back to Jamaica Plain and see if start screen works. Then try updating again.

I have exactly the same thing, also since I installed Kingston.
Sometimes it comes, sometimes it does not, sometimes it just takes half a minute...
Total random behavior.
My PC has no internet connection. Perhaps that is the reason.

Try resetting the cache for the start screen.
 
Open a run prompt by pressing the Windows Key + R 
In the prompt, type %appdata% and press Enter 
Navigate to Cakewalk>SONAR Platinum>StartScreenCache 
Delete the contents of the StartScreenCache folder 
Relaunch SONAR

I had the same issue - Start Screen was blank - no recent projects, no New From Templates entries, no tips, SS wasn't recognizing that the PC was connected to the internet (it was at the time).
 
Followed Keith's cache cleanout directions (but it's StartPageCache, not StartScreenCache).
Start Screen corrected - shows projects, templates populated.
 
Just before the Start Screen was hosed the first time, I was creating some new project templates.
Hosed it again when I changed the default project template then pinned some recent projects.
 
Still seeing some oddball things:
 - Projects don't stay pinned (blue pins are retained while still on the Start Screen but aren't retained when the Start
   Screen is closed/re-opened or restarting Sonar).
 
 - Pinning multiple projects in Icon view hose the List view. List view still shows the Icon view.
   Pinning multiple projects in List view hose the Icon view. Icon view still shows the List view.
   Unpinning everything still has the hosed List/Icon views.
 
Close then open Start Screen "corrects" the List/Icon views (since pin-state is not retained).
Cleaned out StartPageCache a few times, but pin state and List/Icon view thing is still repeatable.  
 

Start Screen error solved.
 
I found two shortcut icons to Sonar on my desktop. When I used the most recent shortcut, everything worked as expected. Most updates in the past have remove previous shortcut icons. 
 
Thanks for all of your suggestions.