the script in my old guestbook-modules show me in this place (gstbk_add.php - Line 43) the include of the ban-module (if avaiable), but i dont have the exact version 2.8.5 here. i've never seen this ban-module, but if you use it in this installation, it is also possible, that the error comes from this module (and not only from the guestbook)
search for this lines in gstbk_add.php (~line 42) and add a double slash (//) in the front of every line, if you work with this module and if you get more errors after the upgrade
if (file_exists(WB_PATH.'/modules/ban/banblock.php')){
include WB_PATH.'/modules/ban/banblock.php';
}