Sonar Professional has issue with Modartt Pianoteq version 6

Sonar Professional has issue with Modartt Pianoteq version 6...
...when I use pianoteq as a VSt2, or VSt3-Instrument. It is shown as a VSt2, and VSt3 in the plugins manager, you can insert the instrument ans work with the track, but when you save, close and reopen that track, a but report is being sent. There's no way to get into that file to change pianoteq's properties, like "insert as VSt2", because Sonar is already going to close down. 
I even cannot try and save that file as .mid, to open it again and correct the settings.
Seems I will report this also to the pianoteq forum for clearance. 
I shall take it out of my plugins settings meanwhile, hoping things will work. (Not, so far.)
My last effort failed as grandiously as all previous ones: In Windows explorer I renamed the file, and changed the file type from .cwp to .mid. I had hoped that, when opening the thing as a .mid file, Sonar would "ignore" the failing plugin, and open the file as .mid anyway. But Sonar crashed again. I will habe to reset my machine to the date before I had taken Pianoteq 6 into my plugins. Hoping I find that very special restore point. Otherwise I'll have to set back to "before installing pianoteq6".
Anyone who as a better idea is welcome to post here.
I'll keep you informed anyway.

Thanks, "scook", meanwhile I did deinstall pianoteq 6, upon reopening my project Sonar was of course showing me the missing plugin. I deleted the (greyed) plugin from the synth rack, then added pianoteq 5, and so far the world is ok again. The guys at pianoteq will have to find out if it is mandatory to first deinstall version 5 before installing version 6 to avoid the described issues.
Of course I could try this out by myself, but they should know about this topic anyway. When I was installing version 6, there was no prompting about deinstalling version 5. Maybe that's where the story ends.

You can choose during install of PTeq 6 what VST types to install. I chose only VST2 and have no trouble at with Pteq 4, 5, and 6 coexisting.

Hi again,
as Sonar kept on crashing when I tried to apply Pianoteq 6, I had opened a support case with them, provided them my last *.dmp-file that had been created by Sonar.
They told me after a week of tough research that they never had seen such a case, however, they offered me a "private edition" (call it  6.01beta, if you prefer) to test.
I repeated all the steps I had previously made, but kept pianoteq 5.8.
After installation of that beta version I rebooted my machine.
I applied that beta version in Sonar, and there were no more crashes, not after closing Sonar and reopening it again with that project, neither when I tried the same stuff with my other application Sibelius 8.6. 
After those first tests I rebooted again and repeated the tests. All remains fine.
So it seems the issue is solved. In case they publish that beta version for all registered Pianoteq users, I shall leave a short notice here.