What are the rules that PIE applies for stacking outputs?

I had a design with stacked inputs in PI Expert that was made earlier this year.
When I log in now, the outputs are separated. When I go into 'Stacking' and select 'AC stack' the selection box turns yellow and I can no longer press Ok.
The checkbox for DC-stack is also disabled.

Is there any information on what is required to stack the inputs in PI Expert?

For information, it's an innoswitch EP solution and I try to create a 12V/4V output.