What brand of anti-virus are you using?
Some are working silently in the background and might flag the trainers after new patch/update, and you'll have to white list them again.
so this happens every time you close the witcher 3 trainer?
I would note that the witcher 3 trainer template actually is over a year old. We've made many changes to the trainer template since then.
While I have not seen this with testing on my own system, errors when closing the trainer usually mean that there is some sort of illegal memory being read or written to. If this is only happening on closing, it is of little concern.
Did you make custom keys for this trainer? Is this repeatable? Are you closing using right click on the taskbar or clicking the red square X at the top right of the trainer (or does it happen either way)? etc.