Gilly,
thanks for your quick response, but I'm sure that I don't truncate/delete catalogs - in fact, this was the first issue I examined when I first noticed this problem; I now checked it once again - both options are disabled.
The point is, that not only old or unused backups dissappear - it seems to be rather accidental. So, for example, from one of our servers today I would be able to restore data from January 6 and 5, and December 28, 23, 20, 19, 18, 17, 14, 1. The same fragmentary (but other dates) list for November, October ...
I think, database settings are also correct; I allow deleting expired data (default settings), but, from my point of view, this only concerns protocols and announcements and should not affect the restore list. And even if - the most recent announcement that is allowed to be deleted is 7 days old - I am not able to restore data that was backuped three days ago.
Harald