Modartt Pianoteq version 6 has issue with Sonar Professional

Modartt Pianoteq version 6 has issue with Sonar Professional  ...
...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.
I shall take it out of my plugins settings meanwhile, hoping things will work. (Not, so far...)

Ok. I thnk you should know this for solving.
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. @ pianoteq: you 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 you 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.
Working with a PC, Windows 10 x64, 16 GB RAM, System HD: SSD 500 GB, Processor: Intel Core i5-3750k CPU @ 3,40 GHz.