I am not sure if it's something else I am doing or a setting but ...
 
1. I Open Cakewalk (Bandlab 64bit)
2. Select a recent project from the list.
3. Mess around.
4. Close it.
5. Click on Existing Project.
6. Cakewalk closes.
 
I can re-open Cakewalk and click on Existing Project and it works.
 
Que?
 
Terry

It's me isn't it. :)

I'm not seeing this behaviour.
 
I've been going through my existing projects, and using the new export module to export mp3 versions of my projects so I can more easily preview them through the media browser.
 
So I'll open up an existing project, export the audio, close it then click on an existing project to open that.
 
I'm not having any issues with Cakewalk closing.
 

Cakewalk closing like that is a crash.  Normally during a crash you'd get a crash report dialog, but it's not uncommon for crashes to be such that the app can't bring up the crash report.  Check to see if you have crash dumps in the directory %APPDATA%\Cakewalk\Cakewalk Core\MiniDumps  (for most users this will be something like c:\Users\USERNAME\AppData\Roaming\Cakewalk\Cakewalk Core  The %APPDATA% should expand in the Window's Explorer to the correct directory, just paste that into the top of the window where the file path is.  Dump files end in .dmp and usually have the date in a MMDDYYYY_HHMMSS like format.
 
If you have a dump file that is dated when it crashed put it on the net somewhere (Google Drive/Dropbox/etc) and post a link here and someone can take a look at it, maybe give you an idea what crashed.  But it's not uncommon for plugins to not clean up very well and crash.  So even if you don't have a dump you should start looking at what plugins you are using in the project you are closing.  One of them is most likely the problem.  Other causes can include device drivers (audio, MIDI, control surfaces).  Updating all your software (OS, Cakewalk, Plugins, Video Card drivers, etc) can sometimes solve these problems too.
 

I would try and see if this happens with a very simple projects. 
 
Otherwise myself I've always used the FILE/OPEN to access my files. I have the start screen disabled. 
I have it set so Cakewalk "opens" in the root of my data drive which only contains my Project folders. So it's only a few clicks to open any project. Because I also have the same file stored on a back up drive I need to know where I am when opening a file. Using the start screen can have you open the wrong version if your not carefull so I don't use it. 
 
I also will use the File menu quick list if I'm working on one song and returning to it the next time I open. 
 

Try waiting a few moments before opening the existing project.
Maybe something is hanging on the save.

I've never seen this before myself.