Same exact thing is happening to me. I got 90000 error then could not log in. Unregistered and suspended on mogstation. This is my 2nd character and I am pretty sure I have never done anything wrong, I always report bots and spammers when I see them. All that I can think of is that I sold all my allagan pieces the other day( I had been doing nothing but collecting these until they were large stacks ) so I have a grand total of like 132k gil after selling all of those. That could be the only reasoning I see because the emails were pretty vague just saying I have breached some terms. I have trained crafting but haven't leveled it yet with one job at 50 and the other at 44. I don't use the markets. I just grind fates and quests. Recieved same sort of insulting termination email as OP. I am calling them today to find out what in the twelve hells is going on.

I use a cryptic password.
I don't use marketboards.
I'm not a high level crafter.
I don't really have an insane amount of gil.
And my FC hasn't seen me online since they locked me out.

As I said this is my 2nd high level character that I have rerolled and I have no reason to associate with gil sellers/buyers or what have you. I only made money through allagan pieces and what quests paid me. None of this makes any sense.