I'm having the same issue.  At first, there was a problem with SessionDrummer (3) not finding its resources during the VST scan...which would cause the scan to fail.  I got that resolved.  I also had to reinstate the plugin in my Plugin Layouts.
 
I'm not sure if there's a problem with the Plugin Layouts also though...
 
Session Drummer 2 was good about identifying/differentiating itself by naming its dll SessionDrummer2.dll.  With SD3, they figured SessionDrummer.dll was good enough.  I have a pile of older projects that still use the ancient MIDI Session Drummer (Sessndm.dll), so it helps my tiny brain to see "Session Drummer 3".  However, it looks like is having issues with simply renaming the plug in the plugin layout.  (I'll have to check this again.)
 
The other thing is that, even though SessionDrummer.dll now scans correctly and clearly runs fine...I have a number of SD3 instances (with multiple stereo outputs) in projects that still open up with the OP's error message.

Is there a problem with opening pre X3 projects with VSTi's with more than a single stereo output?
 

have you tried re installing session drummer 3 using a custom install of sonar x2 ?

chuckebaby
have you tried re installing session drummer 3 using a custom install of sonar x2 ?

I haven't tried that yet...because it will actually start and run in an external standalone host (savihost) and will also work in X3 if I re-instantiate the plugin in the project.  The pain is that I then have to hook up all the outputs by hand and don't remember what kit I was using...and any other setup tweaks I did in the previous instance.
 
I guess what'd really be helpful, is to know what exactly the project is looking for in order to determine whether or not a particular plugin is available.  That is to say...1) what signature is it looking for, 2) where is it located and 3) what conditions would cause that signature to change to the point where it's not considered a match.
 
...edit...
 
Okay...it seems a reinstall was the solution.  (Not that I actually did a reinstall mind you.)
 
The odd bit was that I could bring up an instance of SD3, even though the VST scan had failed.  When I checked the VST properties in the Cakewalk VST\Inventory section of the registry, the uniqueid, vstVersion and wantEvents values were all zero.
 
In HKLM\SOFTWARE\Wow6432Node\Cakewalk Music Software there's a set of registry entries that apparently need to be mirrored in HKLM\SOFTWARE\Cakewalk Music Software.  I copied those over.  Then, in order to force the VST scanner to rescan the dll, I renamed SessionDrummer.dll to SessionDrummer.dll.hide...ran a scan...then renamed it back to .dll and rescanned again.  Finally, it was scanned properly and was recognized in old projects.
 

holy crap.

HooliStudios
holy crap.

...uh...don't worry about all the registry edits.  A chuckebaby's reinstall should clear it up.  The registry nonsense keeps me from having to dig around for the install media.