Thank you for the hint. If this parameter is set to false, there is no "assert" statement generated for this parameter. If the parameter is the only one which is conflicting it solves the problem temporarily. This workaround also lasts only until next click on "Analaze Synthesis Files" in component editor: The _hw.tcl file is generated from scratch so all manual changes are lost.
In my opinion this is still a bug in Qsys: each instance must have own vhd file if instances have changes on parameters for which "assert" statements are generated.