Servers are dropped out of reports

I'm at a customer, who has the folling problem:

when I plan a new report (i.e. "overnight summary") and select all servers to be included in the report, I can save the report schedule. But when I edit it again, some of the servers are not selected anymore. I can tick their checkboxes again and save the report und they get unchecked every time.
This is not depending on the report, but happens on all reports where servers can be selected. And the dropped servers are always the same ones.

Does someone have the slightest idea, what the reason for this behaviour could be?
Any help is appreciated.

Environment: BE Appliance with SP2, other servers are mixed 2003/2008, everything is english.

All the best from Austria,
Oliver