I'm pretty sure this will be suggested at the retro for this event even if we don't say anything, but just in case, suggesting it anyway.
The good: The "ban" mechanism was a blessing in that it allowed accounts to be locked while they were fixed, containing the problem, while allowing almost all players to continue playing.
The bad: The default account-lock message is intimidating, by design. so was disconcerting and scary for users who knew they'd done nothing wrong.
The Suggestion: Allow the account-lock message to be customized by the admin setting the ban, perhaps from an optional "public notes" field, to allow such account locks to be handled in a more user-friendly manner, and even be changed as time goes on to allow status updates to be given to those repeatedly trying to log in.
As a far less-effective alternative, the popup could at least direct users to a (no-login-required) support page, which can be changed faster a client update can be pushed to update the popup. Without a login, this will not be able to give per-user customized info, though.
Benefits: This would allow banned people to see helpful info, like reason and duration of a suspension (if entered), and pointers to the correct appeals process (if any), which could be helpful for reducing the appeals support burden, as well as reducing "why was I banned?" support tickets. For widespread non-punitive issues like the current one, this would smooth the experience of those affected, with knock-on effects to the whole community seeing less of that pain..