Can't Open COM Port

[arduo]

I am using USB Virtual Com Port with CubeMX on a STM32F4 controller.

The problem is, that occasionally, but increasingly often I cannot open the COM Port to my Device. It is very hard to reproduce, as changes in completely different code regions can cause that behaviour. 

The port is enumerated in my device manager, but the connection from a terminal program fails.

What are the possible reasons for this (ram, interrupt-interference, ...)?

Are there any general guidelines for a ''robust'' USB Virtual Com Port?

[AvaTar]

This is usually a problem with the USB driver on the PC side (Windows ?), not releasing resources of the device properly.

Blame ST.

[arduo]

I installed the neseccary drivers. I have to say also that i am using my mcu as a composite device maybe it's not working because of this?