The Fabric Loader itself is too old
Applies to Fabric, Quilt.
What it looks like in your log
Mod 'Fabric API' (fabric-api) requires version 0.16.0 or later of 'Fabric Loader' (fabricloader), but only the wrong version is present: 0.15.11!
What causes it
A mod requires a newer Fabric Loader than the one installed on the server.
How to fix it
Reinstall the Fabric server launcher with a newer loader version.
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Check your own log