I found no word in the manual about Missing Plugin Stand-in. I have a Brainworx Digital V2 that was loaded and playing fine in a project I saved (a set of maybe 10 moves). I I closed and reopened a half hour later, and the plugin stand in came up. I closed and reopened the last version, and the stand-in came up. The plugin is in the folder. I re-scanned and it shows it picks it up, but the stand-in is still there.
 
Is there a way to know where the project thinks the plug is supposed to be?

I don't know what is exactly going on but can you add the plug-in back to the track and see if it sticks?
 
There may be something else going on but try to add it again and see what happens

This kind of thing usually occurs after a plugin has been moved, re-installed or in rare cases after it's been upgraded. It doesn't sound like any of those scenarios apply to you, but it's worth checking out.
 
You can get the full path to the plugin in the Plugin Manager and verify that it matches its physical location.
 
[EDIT] Just remembered another (rare) scenario that can cause this: two plugins with the same CLSID. It doesn't happen often, but I have seen it. This problem will show up after re-scanning plugins.
 
 

Hey, Bitflipper, I don't want to hijack the thread, but can you shed light on why every time I open a project, I get a box asking me to choose between several instances of Boost11? This  seems to be because three different versions of Boost11 have the same CLSID, and I resolve it each time simply by selecting the correct version. It happens even if the project does not contain Boost11. Experimentally, I tried renaming one of the versions from Boost11.dll to Boost11.xxx but the error message continues, simply reporting the new extension. I presume that I should actually have at least two legitimate versions of Boost11 anyway, being 32- and 64-bit, but how do I prevent them from sharing the same CLSID?

Add the ones you don't want to the Excluded Plugins list.

Jonesy, It's a good thought, but I can't - only the one instance appears in Plug-in manager (being the 64-bit one that I want). There is already an excluded entry named Boost11, which references StBoos64.dll. I understand this is some intentional jiggery-pokery undertaken by Sonar itself, and that particular dll is not one of the ones presented by the CLSID "Choose ..." box.