Thank you both @jofemodo and @mheidt for your help!
I thought there was a mistake on my side, maybe a corrupt SD card.
Yesterday I wanted to use my zynthian at a band rehearsal, so I made a system and data backup after the last update.
I wondered why Pianoteq would not start in the evening when it did well in the afternoon…
On my Win7 system I used the following programs:
Xming xming, displays GUI of ZynAddSubFX and Pianoteq engines
PuTTY Windows 64bit
I started Xming first, then I used putty settings found here
Following shell commands came handy:
/zynthian/zynthian-sys/scripts/update_zynthian.sh from here
/sbin/shutdown -r now
was used to reboot zynthian via shell so I had a clean boot process.
Thank you that the solution for this came out so quickly!
God bless, Marius