Hello Dirk,
What model are the servers that get this error ? Could be that the SNMP or WBEM provider can't read the status of the server correctly. Some errors are not seen when WBEM is selected as a data provider in the System Management Homepage. You could try to select SNMP as a data source and see if you can see the error. (you can switch back to WBEM if the problem is solved)Most of the time that's an error in the log part of the server. IML to be exact.
The ILO can only see SNMP events/errors the OSses like Windows can use WBEM.
If that does not help let me know.