Clicking on Existing Project after closing a project causes Cakewalk to close

It's me isn't it. :)

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.
 

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