"plug-in is not present in the plug-ins folder" error

[dg27]

I'm remixing a song I recorded last year. The tracks for this project were mostly frozen.

In four instances, I had used a limiter plug-in on an aux effects track, D16 Group's Frontier.

When I opened the session file, the plug-in was listed as 'not present in the plug-ins folder.'

When I unfroze those four tracks and tried to activate that plug-in, I got the error "The plug-in could not be made active because the plug-in is not present in the plug-ins folder."

The plug-in is indeed in the plug-ins folder.

This is a new machine and a new PT installation. I downloaded and installed new versions of all plug-ins at the time of the install.

When this cropped up, I tried uninstalling the new version (1.2) and reinstalling the version that was used in these tracks (1.0). I thought that maybe PT wasn't seeing it because it's a new version.

I got the same error.

Not a huge problem, but I'm wondering why this might be happening.

I can load a new instance, but it never sees the instances that are there.

Any ideas?

Thanks for any help anyone can provide.

dg