After looking into IIS one of the Application Pools was not started (stopped). Manually started this Application Pool and once again tried to open a Word document
After some time the same error message showed up..
When checking IIS 7.5 the application pool was stopped again.
The application service account which was used :
After adding this account to the local group : WSS_ADMIN_WPG
I started the application pool again and this time it kept running.