I automated the workaround pointed by Nick Phillips, within the attached python script: it successfully corrects the reported error.
> For a workaround, either set the template_dir option in the [options] > section of your viewvc.conf and comment out all of the explicit > templates specified in the [templates] section Please note that Ubuntu seems to have the same problem when updating from viewcvs feisty to viewvc gutsy: https://bugs.edge.launchpad.net/ubuntu/+source/viewvc/+bug/152438
viewvc-template
Description: viewvc template fix
-- bou ^
