Problem locating altera_avalon_pio_regs.h

Hello,

Since half an hour ago, the Nios IDE is driving me crazy. Whenever I try to create even the simplest project (like the counting binary from the hw tutorial), during the compilation it fails with: "unable to find include file altera_avalon_pio_regs.h".

I tried everything - deleting the whole workspace and all my projects, and starting the IDE "anew", closely following the instructions in the hw tutorial. No avail.

I didn't delete anything, and the aforementioned file is live and healthy in its directory under the altera/nios2 tree. It even appears in the "Nios device drivers" folder in the Project view.

Has anyone experienced anything similar ?

Thanks in advance