After resisting the urge for many months, I just updated Cakewalk's RXP player via the command centre. Now when I open a new instance I see no files ... the groove browser is empty! (Click pictures to enlarge)

When I open old projects I get a notice about missing rxp files ...
 
However, here is the strange part, when I search my computer for the particular missing file (above) I find it under "Drop Zone" content. Can someone please help me make sense of this. How do I get the rxp content to appear in the rxp groove browser on startup?
 

RXP has a registry entry which should be pointing to its content. The entry is
HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\RXP\Contents Folder
The default value for 64bit SONAR is
C:\Program Files\Cakewalk\Vstplugins\RXP
In this path should be a folder called Contents which contains a file "programs.lst" and a collection of more folders. In my case I see three folders "Loopmasters," "Sample Magic" and "Smart Loops." These three folders hold the Rex files. Note, this agrees with the path in the "Multisamples Not Found" error in your post.
 
Since RXP has nothing in the browser it may be the registry is not pointing to the real location containing the Content folder. Possibly this is not a default installation and the installer got confused somehow. The programs.lst file could be bad, in which case deleting it and starting RXP again should rebuild it. Or the samples are just not there.
 
FWIW, there is some overlap between the Dropzone and RXP content. However the two synths do not use the sample location for their samples.
 

Thanks Scook for your detailed help!
The registry is not my natural domain, but on the weekend I'll have a look at where you are pointing me.