Access Denied at building navigation

I'm on Encore CS5, Windows7 64bit, very fast PC. Trying to burn onto DVD+R DL 8.5GB 8x speed disk.

I'm trying to create a DVD with no menu - just one video file to play once and stop.

I check the project but there are no items found so I click on "Build".

It seems to go fine until "Building navigation", at which point a window pops up with "Access denied" and it stops.

No data is written to the DVD+R disk.

This happens whether I import an MP4 or a Premiere project (using Dynamic Link).

I'm totally new to Encore.

Does anyone know what the problem could be and how to fix it?

Johnny