1. When I start Darkstone I get an error message that there is no more hard disk space although there is far space on the partition more then the required 70 MB. But the game starts as usual.

2. When I use the Astral Hand on the pyramid in the town at the end I get a message that I have won but then I can't get back in the main menu. The game don't respond to any input anymore.

Does anyone know what a solution for this problems could be?

Definitely send something to support. They'll likely recommend a few things:

1) Make sure it's not installed in C:\Program Files\ or any other protected directory.
2) You will likely want to redownload and reinstall. A small glitch can sometimes be introduced from a bad packet that wasn't caught in the checksum.
3) There may be a movie at the end that is causing the problem. Are you able to run the intro movie? Have you disabled the movies in an .ini somehow?

I hope you get it working. It's a fun one to play through a few times. Of course, you could probably just start over with a higher difficulty and use the same old characters.

Just tried it again and strangely it ran without any problems.
But thanks for you help anyway. :)