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Separate names with a comma.
1.16.4 is the same protocol as 1.16.5
I believe this is a viarewind bug in relation to bossbars - thread for the issue https://github.com/ViaVersion/ViaRewind/issues/249, I...
Do you use BungeeCord? Have you updated it?
Do any other versions work?
Otherwise provide /viaversion dump
and your server version is?