Can't find design entity in project navigator

Hi,

I have recently un-earthed an old design which was originally synthesized using Quartus II 5.1 and targets a Cyclone I (EP1C4), on which I want to add new features.

Since we are not changing the hardware, it will have to remain the same EP1C4, but I wanted to port the design to Quartus II 11.1 SP1.

After doing some adjustments to make it compile correctly, the whole Analysis, Synthesis and P&R now works OK. But:

When I double-clic on a design entity in the Project Navigator ("Hierarchy"-Tab), I get the message "can't find design entity xxx".

Every other means of calling a design file works ok, and it also works after explicitly running "Start Hierarchy Elaboration".

How can I change this behavior?

Thanks,

Alex