(Solution is in post #14).
 
I have a new computer with new drive letters.
I reinstalled everything as the old computer, but drive letters are not the same. May be some plug-ins are in other folders too.
The problem now is that when I try to open an old project, Sonar accuses the error below:

 
In this case, Altiverb is installed, in drive "C:" and before it was in drive "H:".
How to solve this?

Anyone?

Hey there - after reinstalling, did you add the new location to your plugin paths in preferences?

Delete the plugin reference in the project and load a current one in its place. save it and the message will no longer appear. 

Yes, I added the new location when I reinstalled the plugin.
If I delete the plugin reference in project, I'll lose all configuration changes I did in that instance. Also, when deleting a plugin, Sonar deletes all envelop automations that I have inserted of this plugin.
Any way to solve this without deleting it?

The plugin will need to be in a folder Sonar expects it to be in for Sonar to find it. You could add that location as another VST folder to scan in Plugin manager. 

John
The plugin will need to be in a folder Sonar expects it to be in for Sonar to find it. You could add that location as another VST folder to scan in Plugin manager. 

The plugin is correctly installed and working on C: (for new projects), but as I said, my old project is referring this plugin's instance as H: (old drive letter). I don't have H: anymore.
So, in my old project I have a "H:" reference that can't be changed nevermore?

This is where preset mgmt falls down a little bit. It would be awesome if you could try and force the preset settings in cases where the substitute is identical.

I'm out of answers for this issue. I wish I could be of more help but nothing I can think of would be worth the effort. Sorry.

First, does it show up in your plugin list?   If so, try adding it to a track just to be sure it still works.   I had exact error and fixed it today (with Altiverb too!)   It needs to be in the same place it was before.   Today, I just copied the DLL  from Cakewalk vstplugins to steinberg/VSTplugins.  That did it.   (It actually is still in 2 places but it works with my old projects and loads fine for new. I haven't tried to get rid of the old one in cakewalk yet but for now it ain't broke so Im not fixin it anymore