I've recently picked up a UA Apollo Twin Mk 2 (thunderbolt) for PC and when in Sonar it works fine for audio.. however, if I add a UAD2 plugin to a track.. it causes an error that states the "Audio engine has stopped." I am able to resume working after the error like nothing happned.. just didn't know what was causing this..  
 
For what it's worth.. I am using the Twin Mk2 as my interface.. it is a DUO.. and I have another DUO and a SOLO installed in my PC via PCIe. 
 
Thanks
 

Are you trying to add a UAD plug-in during playback when this happens? If so, probably related to SONAR's PDC mechanism kicking in to keep everything sample sync accurate. Some DAW's like SONAR are more sensitive to dropping PDC inducing plugins into a project on the fly during playback. I typically add any UAD plug-ins with playback stopped to avoid this type of error and all is good.
 
Cheers,
Billy Buck

Billy Buck
Are you trying to add a UAD plug-in during playback when this happens? If so, probably related to SONAR's PDC mechanism kicking in to keep everything sample sync accurate. Some DAW's like SONAR are more sensitive to dropping PDC inducing plugins into a project on the fly during playback. I typically add any UAD plug-ins with playback stopped to avoid this type of error and all is good.
 
Cheers,
Billy Buck

Same here. Some of those UAD plugs use a LOT of DSP, which is why it's a good thing they don't rely on the host CPU for processing. The Manley EQ in particularly sounds so good because it's gorging itself on processing cycles.

Actually I hardly ever insert plugins while playing.. in this case.. each time I added the 88RS channel strip (while the transport was stopped).. it woudl give me the said error. It would pick back up right after by hitting play again.. but it would "crash" the audio engine each time. This happens on the first instance of the 88RS (the new "strip collection" version).. or the last one (once I run out of DSP).. if I use another plugin (say the "legacy" 88RS version) it doesn't. 
 
I know previous versions of the UAD software had a setting for Sonar.. but they no longer do.. Just throwing this out there in case it's a bug.. that can be fixed if reported. 
 
Thanks