After Core upgrade to 1.12 we get:
ERROR: at line 127 in file
/www/htdocs/xxx/lib/smarty/libs/sysplugins/smarty_internal_templatebase.php:
Unable to load template file
'/www/htdocs/xxx/modules/Guestbook/templates/admin_tab_entries.tpl'

Afaik it isnt possible anymore to load templates within the modules folder with
full path included.
This behavior changed because it is a potential security risk. But I will get
back to you on this!

Grtz. Rolf


ps. All module templates should be placed in folder
/modules/Guestbook/templates/

You can call them from the files with:
echo $this->ProcessTemplate('template_name.tpl');