This does not appear to be a stacktrace of version 4.3.0.. can you verify? If this is by any chance Mendix 4.3.1 then please move to 4.3.2
Updated: It seems your new users do not have a language for some reason. This shouldn't be possible as there is always a default language in a project. Can you file a ticket with a reproduction of this case? Please mention this forum question if you do.
And have you tried this with a newer version yet?