FrameMaker 2019 Cross-references

Hey Community

I have the problem that when opening an existing xml file, the cross references are gone. I can select the cross-reference, save it and it can be seen in the structure view and in the text. After closing the xml file and opening it again, the cross-reference is gone. The xml is saved as a ditamap. I have FrameMaker 2019 version 16.0.4.1062 and Windows version 10. 

Many thank's you for your help.

Dorotea