As for current condition, bossworld is down for maintenance and such.
When this happens, usually you will always get kicked to hub whenever trying to enter bossworld's portal and error message that your data can't be saved.
So please wait patiently until the devs finished the fixes or updates and it will be accessible again.
Just in case,
If there is condition where only you that are unable to access bossworld, some possible solution:
1. Use 1.8.9 as it's less glitchy
2. Try rejoining or relogging
3. Keep trying to join the portal (There was a such bug, and after trying multiple times it will work)
Or make a bug report if that still happens
Hope this helps.
