I did a quick search on the error – and it seems a lot of people are mentioning taxonomies, and sometimes menus… I'm guessing yours is taxonomy because other pages – with the same menu structure – show fine. Here is an example:
http://wordpress.org/support/topic/catchable-fatal-error-object-of-class-wp_error-could-not-be-converted-to-string-2
found possibilities by searching "object of class wp_error could not be converted to string" – for what it's worth, and many possible approaches come up… including adding var_dump(…) before the line to find out what is happening.