Players are being kicked for flying
What it looks like in your log
Flying is not enabled on this server
What causes it
The vanilla anti-fly check is on (allow-flight=false) and something let a player fly: elytra, a jetpack, a mod that grants creative flight, or plain lag that looks like flight to the server.
How to fix it
Set allow-flight=true in server.properties and restart. On any modded server that is the correct setting; the check predates mods and cannot tell a jetpack from a cheat. Use a proper anti-cheat plugin if you need the protection.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log