This is so weird.  I seriously can't dock anything.  I opened a completely blank project in Platinum and there has to be something I'm missing.
 
I went into X3 & created a blank project and of course everything works normally.   ??? 

Hold down Ctrl key
 
 
Though 32 bit soft instruments seem to have a bug and are not docking. 

Thanks aMuze.  How did I miss that change?  Hrmph. 
 
I too have had that problem with 32 bit synths in the past, but I don't think I use any now.

Right click on the top bar of your soft synth and it will ask if you'd like to doc.

And thanks Jimbo. 

Maybe has something to do with plugin window recycling?
 

32bit plug-ins in 64bit SONAR dock by the VST header instead of the bridged plug-in UI. To display the standard VST header in Platinum hold the ALT key while opening the plug-in UI. The header will be behind the plug-in UI. Previous versions of SONAR use SHIFT while opening the plug-in UI.