Our trusted vet did all the checks possible. There is no obvious cause for this accident. The foal (a colt) was complete, well - formed, with no visible faults. The mare has no obvious faults as well. We will know more (or not...) when the blood tests will be completed - next week.
We have had no movements in the herd. No new horse has arrived since almost 2 years. I would be utterly surprised if it would be a herpesvirus then.