The exact same thing happened to me. This was the Trello support team response:
"I've taken a look at your account, and ultimately, the problem is that the email address of your former employer was still attached to the Trello account. In their recent account claim, this triggered your employer to claim ownership of the Trello account, which is something Trello's terms allow Enterprises to do. Because the email address was still on the account, your employer identified it as an account that they should own, and ownership of this Trello account was transferred to your former employer, so no changes can be made to the account, and the company owns that account.
It sounds like you have personal content in this account that you want access to? Given the account ownership, that's not something that we can do on our end, unfortunately. If the company consented, they could remove your account from all company teams, and then we could remove the Enterprise association, but that's something you'd need to explore with them, if they'd be willing to do that."