Respectfully, I apologise, but @NotHereToPlayGames has no way of knowing what was or wasn't done or changed in Supermium at all. All what he wrote here is speculations, not facts. And how does he know that any modifications were done to the Java engine of the browser at all? It's Java what is not working for the login window, I'm pretty sure you perfectly understand that. There was no need to meddle with the Java engine just to make the browser work on XP. It's nonsense. So far, all facts, including the ability to log in from oldish versions, similar imports to the v.124, non- working sandbox, etc. all point out to the outdated browser core, and @NotHereToPlayGames can't prove otherwise.
With huge respect.