AlbertWeijers' issue could be caused by other things for sure, esp. if it carries over to other vocal tracks.
Others point to good things to try out.
Here's also a good resource for anyone to use to trouble-shoot audio issues further..
Glitch Free - by Brad from Cantabile software
Has well organized and easy to understand info and solutions (like settings from CPU or page file, other background processes etc.. and so many things - a lot of those maybe not so necessary on newer powerful machines).
Always a good publication to share around.
Mismatched sample rate glitches is not a thing particular to Pianoteq - but I've encountered a mismatch glitch or two.
If you think it's this.. maybe in the DAW delete (or refresh) plugin cache, and set the DAW to re-scan plugins on startup.. that way, you may encounter less 'old' data getting in the way of normal functioning.
I know some people who never seem to have issues - but I'm someone who has - so I understand when some stupidly simple nonsense can take way longer than normal to find and fix.
Good luck Albert! Let us know how you got on if you get time.
Cheers all.
Pianoteq Studio Bundle (Pro plus all instruments) - Kawai MP11 digital piano - Yamaha HS8 monitors