@ N-drju: Just the specs as I can see it at first glance:
Win10 64 Bit, Intel Core i7-4790 @3.60 Ghz, 16,0 GTB RAM, Nvidia GForce GTX 760
@ Oshyan: Thanks for your suggestion with the tgdcli.exe - I hope I don't forget before the next try.
When the crash happended I just got an error message like "Terragen doesn't work any more - Windows will inform you if we find a solution" No Log - no other message.
I tried 10-15 times including one "Repair-installation of the latest TG4 version"
As well I tried starting from Desktop symbol, taskbar, from the program folder and by just opening a tgd file.
Always the same: The program window opened and while trying to show the TG user interface, it crashed.
Graphics driver is the latest version.
The Eventlogs don't show any crash!
(since 04/01 - and that Terragen crash was plausible because I accidently had overrided the size of subdiv cache with some million GB
Since the Rollback I had no more problems - so at the moment it's ok for me to keep the OS on the last Windows version because this PC doesn't do anything else but rendering TG animations @ 7/24