This one is driving me crazy...
 
In short:
New project will load up all my plug-ins, no problem...VST2 or VST3.
Some older projects will occasionally come up with the notorious "VST Plug-In Error" prompt...failed to load...(8000ffff) if I try to load newer VSTs.  I've rescanned every way possible.  I've even completely uninstalled SONAR following the directions https://www.cakewalk.com/Support/Knowledge-Base/2007013392/SONAR-2015-Clean-Install-Instructions
 
No matter what I do, I simply cannot get an older project...and we're talking a few months ago...to load up VST plugs that were not on the system when I began the project (at least, that is my suspicion).  Two that come to mind are Ozone 8 and FabFilter-R
 
It's like the projects have a certain path they look for to find plug-ins, which is determined when the project is started.
 
Any ideas?!  I guess I could try to re-install the offending plugs, but like I said, I have no problem whatsoever using them in a NEW project.  Thanks in advance.

They aren't by any chance old 32bit DX plugins ?
 
32bit DX plugins have different ID's to their 64bit counterparts, and a 32bit DX plugin can't be loaded by a 64 bit host.

No it's not that.  These are shiny new 64-bit VSTs, both 2 and 3.
 
Within seconds, I could try to load it onto a certain project, get the VST error, close the project, start a new one, and load the same plug-in no problem.
 
The issue seems to be project-specific, which is the mind-boggling part.
 
I'm about to export or copy/paste tracks from the offending project into another new one just so I can use the VSTs... =(

I’m not sure which version of Sonar you’re using but in the Browser window if you right click the plugin icon make sure you’re use ng the default plugin layout.