OS? Version of ID?
This means that either you have an extra Helvetica Medium font installed and it's interfering with ID finding the T1 version, or the T1 version is damaged. I found this really old thread that explains it fairly well: http://www.asadesigner.com/22-indesign/2d812de56777cc90.htm
The good news is that it's probably only affecting the page information marks.