Hello Paul, I think the problem is indeed the board style. The board style is an attribute of the user (i.e. the user can chose how to see the board). Currently there are two options available to chose from: "prosilver" and "Metro Teal". But "Metro Teal" seems not to exist. (maybe it has been deleted accidentally)
So, as soon as a user switches the board style to "Metro Teal" he will be kicked out and cannot log on any more. (and, thus, also not switch back to "prosilver")
All the old accounts (created before ~ 2020) have "Metro Teal" and can't log on therefore anymore. (The notable exception is GalFisk

)
In my opinion, the solution would be:
- either: set the board style for all users to "prosilver" and remove "Metro Teal" from the selection list, if this is possible & make sure that new accounts are created with "prosilver" by default
- or: create a board style named "Metro Teal" and see whether this revives the old accounts