[MC-173176] Serialization error affecting operators list and whitelist

The bug

  [20:52:28] [Server thread/WARN]: Failed to save operators list: 
java.lang.IllegalArgumentException: class zu declares multiple JSON fields named a
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:170) ~[server.jar:?]
        […]
        at zw.f(SourceFile:114) ~[server.jar:?]
        at wg.B(SourceFile:114) [server.jar:?]
        at wg.<init>(SourceFile:31) [server.jar:?]
        at wh.d(SourceFile:186) [server.jar:?]
        at net.minecraft.server.MinecraftServer.run(SourceFile:634) [server.jar:?]
        at java.lang.Thread.run(Thread.java:748) [?:1.8.0_222]

Original Description

So i try to add players as op on my server and it says the following message: An unexpected error occurred trying to execute that command
and when i try again it says: Nothing changed. The player already is an operator.

But the problem is that the op text file is empty