drupal_is_front_page undefined after instal?

Hi

We just installed Drupal yesterday, and applied available upgrades (now at 4.7.3).

We were able to log out and back in...

Later, after changing a few admin items, tried to log out and got

Fatal error: Call to undefined function: drupal_is_front_page() in
/home/kfeuerhe/public_html/cms/themes/engines/phptemplate/phptemplate.engine
on line 142

We didn't create any aliases afaics... and now we can't get back in: this is what happens....

What's up? How to fix?

Thanks