Ok, I try to describe more detailed.
Our app creates a number of VCOMs (e.g. 3) at startup, using always the same port numbers. When closed, our app cleans up and removes those VCOMs properly.
If a machine crash occurs, obviously no cleanup is possible and maybe used temporary files, registry entries or whatever can't be disposed of properly and will survive a system reboot and the following launch of our app.
After such an ungracefully shutdown and start of our app, the VCOMs are created successfully and visible in Device Manager. They can be opened by other applications (e.g. Hyperterminal) successfully.
The problem symptom is, that no data is received, despite of all successfull operations so far. The VCOMs are present, but deaf and dumb, if you want to say so.