My minecraft server keeps kicking all players and they can't connect back for about 10 seconds.

Upload your latest.log to https://mclo.gs

Try to see if the issue persists without Krypton since it modifies network and packet related stuff
Your server also seems to be lagging (spark report may be helpful) but not sure if it's related to this specific issue

Removing krypton doesn't help

I'd highly recommend updating to 1.21.10 first and seeing if that fixes the issue, if not try with all mods disabled
1.21.9 was sort of a transient version that's no longer supported or being actively developed on