We located the problem ourselves.

In the report, we have a BeforeReport function.
In this function, we call a report Packaged Function m_init.f_init(... ) that itselfs calls a db packaged function.

When we create a report function ( not in a package ) f_init, instead of the packaged function, and we use this function in the BeforeReport function, everything seems to be OK.

Looks like a bug in reports to me.
I will keep you informed about what oracle support has to say.

Regards
Gert