I kind of guess this is in the pagelayout or related templates -> You are not supposed to use $node in other templates then node templates. This is because the variable is only there (in pagelayout and included templates) when view caching is disabled or if the page you are viewing is not cached yet (no view cache for that node generated yet).
Edit: This is a long standing misunderstanding introduced at the time view cache was introduced to 3.x, this is fixed in 3.9 by removing the $node variable from pagelayout completely.