Invalid Password?

A public game has absolutely nothing to do with the passwords. It only means that anyone who knows the game URL can access it (without being logged into their Forge account and having received an invitation link to the game).
On the contrary, if you set the game to public, you really should have a strong password for every player, otherwise any random person on the internet could just join the game and delete all your work.
See explanation of what Public vs Private means here : Public & Private Games - Docs - The Forge

As for the user passwords, you set that up in Foundry itself when you login as DM and go under the “Configure Users” or “User Management” buttons, you can create/delete users and set their passwords.
If you don’t remember your GM password, you can always reset it, see here for more information : Reset your Foundry VTT Game/World Password - Docs - The Forge

I went through this with a couple of guys on the Discord channel. And the general consensus what the game was corrupted in some way.
I created a new game and exported out the stuff from the old game, getting into it by allowing Forge to Manage users (no password required, private)

I had a look at the FAQ above just for the sake of it, same result “Invalid Password”. When I enter the game and Edit World there is No Reset password button|
no user
But doesnt really matter now but would be nice to know in case it happens again

I just answered you on Discord, but yeah, the Edit World dialog that has the option to reset passwords is only available from Foundry Setup screen if the Game Manager is disabled. In your case though, with the Game Manager enabled, you’d click the “Configure Players” button in (selecting the tab of your game) and click the Reset Passwords button from there.
If you enable or disable the User Manager option while Foundry is running, that can corrupt the passwords, it’s a known issue since v9.