Warning: One or more variables are no longer available

Since the update to ONE DATA 5.0.0 our Production Line shows a warning saying

One or more variables are no longer available.

This sentence is repeated a second time as mouseover text on the navigation box to variables.

Does anyone know what to do? What action should I take as user? It looks like all variables I have set in this PL are also shown correctly in the UI. Will the results of my Production Line be valid?