in one project, when I right-click on a clip, I'm getting a "Locate VST plugin" error dialog, for a plugin which I have not used in this project (not installed on the computer at all).
 
After I cancel the dialog, I get normal right-click behavior.
 
Very odd.

it's also happening in fx bins. 
 
Everything was acting normal.  Then I believe I was right-click selecting a group of clips, the automation data also got selected.  I moved everything without realizing the automation data was selected, then undid, then when I reselected the clips I got this obnoxious behavior.  I restarted the computer but it continues to act funny.
 
 
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and now Sonar seems to have decided it has found the missing plugin, though it still doesn't show up in any plugin lists within Sonar... weird

hmmm... now it's started up again in a new project.  I don't understand why Sonar has decided it needs to find some plugin which I certainly haven't ever used on any of these projects.  Yesterday I opened a blank project, right clicked in the fx bin, and got the same "lost plugin" dialog.  But doing that seems to be the workaround.  Super obnoxious behavior though....
 
 
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no, that only is avoiding the problem in the track fx bins, bus bins and audio data right clicks still pop up the lost VST dialog.

Is this a custom project template? Does the problem occur using the factory supplied Basic.cwt?

OK, it has to do with the plugins which show up in the plugin manager as greyed-out.  I found a place in a nested menu where the offending plugin was showing up, removed it, now the problem's gone. 
 
 
Now I see that a 32bit VST I was happily using just a couple of days ago has gone grey in the manager and is no longer available to use....