I have some ideas.

Looking at the logs as Tim mentioned.

Did you try a clean re-build of forrest?
cd main; build clean; build

Also in your project: 'forrest clean'
just in case.

The "PDF output" plugin was updated recently.
Depending on how old your 0.9-dev trunk SVN was,
that might need local deploy.
IIRC, the cleaning mentioned above makes sure.

By the way, as we are discussing the development
version of Forrest, it is better on the "dev" list.

Anyway, hope that helps.