I'm running Sonar Platinum, I also still have Sonar X3 and Sonar 8.5 still installed on my system.  I can insert RXP using sonar 8.5 but when I try to insert using Sonar Platinum, or X3 I get the error "Failed to insert soft synth. The synth may not be properly installed" error

Works OK for me. Did you rescan your VSTs?  Did you look in the Plug-in manager to make sure it's listed there?

Yes it's listed, but when I try to insert it gives me the error "Failed to insert soft synth. The synth may not be properly installed" Do you have to reinstall Sonar just to reinstall RXP?  It does recognize it during a vst can.

Could try that. First move the existing RXP folder out of the way.
 
The default location for a 64bit install is "C:\Program Files\Cakewalk\Vstplugins\RXP"
For 32bit install "C:\Program Files (x86)\Cakewalk\Vstplugins\RXP"
 
Use either the advanced install option with X3 and select the plug-in or reinstall the "Platinum Instrument Collection"
 
If that fails, try running SONAR as administrator, although it should not be necessary. The plug-in should run OK without elevated privileges.

Ok, I will give it a try....
 
Thanks...

well I tried removing Vstplugins\RXP and reinstalling using "Platinum Instrument Collection".  Still done the same thing. but I only checked the 64bit option when I ran "Platinum Instrument Collection" installation.  So I went back and selected the 32bit option.  Now in Platinum and X3 when I looked under my tabs RXP was only showing up previously under my DirectX Tab. But now it also shows up in the VST2 Tab but it reads RXP(32bit).  If I chose the 32bit it works, but It will not work when I chose the 64bit.
 
Keith

well I tried removing Vstplugins\RXP and reinstalling using "Platinum Instrument Collection".  Still done the same thing. but I only checked the 64bit option when I ran "Platinum Instrument Collection" installation.  So I went back and selected the 32bit option.  Now in Platinum and X3 when I looked under my tabs RXP was only showing up previously under my DirectX Tab. But now it also shows up in the VST2 Tab but it reads RXP(32bit).  If I chose the 32bit it works, but It will not work when I chose the 64bit.
 
Keith

The RXP plug-in including with X3 and Platinum is a 32bit VST plug-in. There is no 64bit version of this plug-in. A 64bit install will install the 32bit VST plug-in. The difference between a 32bit and 64bit install is the location of the plug-in. I do not have 8.5 installed but it sounds like it might have been a 32bit DX plug-in in that release. This should not even show up in 64bit hosts. If it does, the error you are seeing would make sense.
 
The 32bit VST should work all of your hosts. The 32bit DX plug-in will only work in 32bit hosts.
 
I am not clear if the problem is resolved for your 64bit hosts or not.

Well the RXP showed up under the DirectX menu tab, but I never used it until now with a new project I'm working on.  I guess when I installed X3 and Platinum when It got to installing the VST's I choose 64bit. I don't know why RXP installed under the C:\Program Files\Cakewalk\Vstplugins location but it did. It's not in the C:\Program Files (x86)\Cakewalk\Vstplugins location.  Even after I did the 32bit install the RXP Folder is still installed in the C:\Program Files\Cakewalk\Vstplugins but now it shows that it's only a 32bit VST. and it shows up in the VST2 Menu Tab.  But when I go to the DirectX menu Tab RXP is there and it doesn't show it as a 32bit.  
 
Strange....

32bit VST plug-ins are put in the 64bit path if the 64bit installation happens first. Once installed, I am not sure they get installed again in the 32bit path. I have not installed 32bit SONAR in some time.
 
I do not recall what the plug-in format was prior to the X-series. I only mentioned DX because of your posts. I know RXP was never ported to 64bit regardless of format.