The problem seems to be with the words in the URL:

https://forum.square-enix.com/ffxiv/...public-content - Forbidden Access 403
https://forum.square-enix.com/ffxiv/...s/519359-aaaaa - works just fine

https://forum.square-enix.com/ffxiv/...een-a-few-days - Forbidden Access 403
https://forum.square-enix.com/ffxiv/threads/518903-b - works just fine

I don't know what kind of issue is happening in the backend but some words are really triggering the forbidden access situation.