Thank you for the info. I'm still using V1. Which isn't bad ad all :)

 I got a message from Zplane that they have figured out a solution for this issue.
 
Go to plugin manager, open the plug-in properties of Elastique Pitch V2 (from the list of all registered VST2 plugins in the middle, not from the list of your preferred ones on the right) and set the number of max used inputs to 2 (instead of 8).
 
In the picture it says "Channel Tools" but ignore that. It should say "ElastiquePitchV2" or so.
 

Thanks for posting this. Great!

Thanks for the information and keeping track. Nice that we can use it now. But for me it seems to be more a work around than a bug fix. The channels which are not in use are causing the feedback. This setting restricts Sonar only to use the set amount of channels. I checked it with a surround channel. If you set the input to 8 you got the feedback, if you set it to 6 it’s working fine.
 
Do you know if they are working on a final solution? In v1 the max inputs is set to 8 and it don’t made any problems.

I presume they will fix it in an update yet they did not say that in the message I got.

I've been in touch with the ZPlane developers and sent them some information regarding this issue. The problem was caused because SONAR uses all the inputs available as stereo pairs. The plugin manager settings in the post above can be used as a workaround but Zplane will be investigating a fix from their end that is more optimal.

This from Zplane:
 
We finally figured it out and for the next release (not the 2.0.4, which went online today, but the next) there will be a build-in fix.

I just noticed that 2.04 is online and that the bug isn't fixed. But good news that they take care of it.

Version 2.05 is out. There is nothing mentioned in the release notes about a fixed "Sonar" bug, but it seems that the bug has been  fixed.

finalymetten
Version 2.05 is out. There is nothing mentioned in the release notes about a fixed "Sonar" bug, but it seems that the bug has been  fixed.

I guess they are referring to the very problem within Sonar when they say "fixed: cross talk in multi-channel setups under certain conditions".
 
Good news!