Striped background on workspace

This is very odd. In the last couple of weeks I've been working with my X8 on a new computer and everything seems OK but on the first project I worked on a striped (actually more liked lined paper) BG shows on the working area, i.e. the page itself.

The JPG with the graphic shows how the lines show through a transparent BG. The second JPG is the blank page with the lines. I've raised the contrast quite a bit to show better what I'm seeing.

Now, when I go to preparation for printing and convert to CMYK JPG, the lines disappear. So they are not "really" there, if you see what I mean. I've checked in the options, but I can't find anything that would explain this. BTW it hasn't happened a second time (yet).

Anyone got an idea?

Karen