It looks like you uploaded the files to the wrong directory and written over a stock file needed by and included with xenforo.
Try this...
Find the .zip with your copy of xenforo.
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extract it and go to the following directory...
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/library/XenForo/Model ,then find Post.php
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Copy that file and upload it to your server inside the following directory - /library/XenForo/Model/
After re-uploading that file and overwriting the existing one in that folder...check that page that threw you the error before.
If you don't have that error anymore you only need to upload the addon to the proper directory...
extract the zip containing the addon from this thread and upload the entire PostReplace folder to the library folder on your server.
Let me know if that solves the issue.
Quick Find:
Regular expression:
Code:
#\[url]https://www\.facebook\.com/photo\.php\?v=([0-9]+)(?:[^\[]+)?\[/url]#siu
Replacement String:
Code:
[media=facebook]\1[/media]
Quick Find:
Regular expression:
Code:
#\[video=dailymotion;([^]]+)][^\[]+\[/video]#siu
Replacement String:
Code:
[media=dailymotion]\1[/media]