Hello,
I am working on this model and out of nowhere I see these Red cross on some elements in some diagrams. Do you guys know what might have caused this and how to fix it?
Thanks in advance.
Hi,
The red crosses appear on diagrams that are not synchronized and not refreshed from the model. (see Unsynchronize your diagram and Diagram Refreshing Strategies)
They indicates that elements that are represented in the diagram no longer exist on the model (they have been deleted somehow) and the diagram is still reflecting the previous state of the model.
You can refresh the diagram using the 
button.
There is no way to retrieve the model element behind, except by restoring them from a previous version of the model, or recreate them manually. (see Git manual if model was stored on git)
Regards,
Philippe