lost references for a derived sketch Inventor 2013

[LotharBoekels3623]

Hello,

yesterday was the first time I detected it:

Have a layout-file and derive geometry and a sketch  into a part file.

So far everything works fine.

Later I came back to the already finished file and wanted to change the holes that were controlled by the sketch and:

All the references to the layout file got lost. Every piece of geometry in the sketch is no more constrained.

In the derive-dialog box everything is fine. Even if I rename the scetch in the layout-file it is renamed in the part-file. But I cannot replace it. Cannot delete it without loosing lots of hours.

This is not only in one file. I detected the same thing in lots of files. A desaster! Do not know when it started an no idea which event started this.

Any suggestions?