Users are given Editor access. So they can create documents. The forms do not have any access control set. There is no script or formula that would cause any such effect.
Also no document versioning.
As I said before, this happens only for some documents, not for all of them. This also can't be a matter of a design change where older documents had some sort of security set, which no longer applies for newer once. The database is archived frequently, documents are never older than a few month.
I can't make any sense of it. I'm working with Lotus Notes since 1995 and never experienced such a problem. Could it be a database corruption?