What the message is telling you is that there is a missing file: /mod/forum/lib.php
So, if you look on the server that file will not be there (or if it is, it may have improper permissions or some other issue where it can't be opened by the webserver correctly.)
You should look on the server in the /mod/forum directories and see what files are there, and then compare that to what should be there (you can simply download a copy of the Moodle zip file for that version, unzip it on your local drive and take a look.)