I'm going to do a wild stab in the dark here and ask, is the character you want to use with the forum on another seperate FFXIV account? The reason I ask is a lot of times when players cannot access certain characters for whatever reason is it turns out their 'current' character is on a new FFXIV account they inadvertently created instead of the earlier one (this was particularly common with players from 1.0 who forgot their old account details and so created new accounts, thus they couldn't use their old character data with their new account). It also commonly happens with players who first create a character on one version of the game (like PC), and then purchase a new version (like PS4) and inadvertently create a new SE account thinking they need to do so to play.

Of course that may not be the case here, but it's worth checking.