I tested a few databases with gfix:

        Number of record level errors   : 1

        Number of record level errors   : 2

So there are problems in the database. I have added a feature in my software for the usage and termination registration. It seems al normal. So no bad behaviour or power failures. Maybe it is the hardware. But suddenly a large increase of problems with a few different customers puts my suspicion also on the latest Firebird 2.5.8 self.