Dear @Bart.Klootwijk,
I can’t review your SubDyn input file now, but when you enable both PtfmYDOF and YawDOF in ElastoDyn, it is necessary to set PtfrmYIner nonzero as discussed in the following forum topic: Wind Veer Parameter - #55 by Jason.Jonkman. A model would likely go unstable without that.
Best regards,