--- a/net/minecraft/server/players/StoredUserList.java
+++ b/net/minecraft/server/players/StoredUserList.java
@@ -97,6 +_,7 @@
     }
 
     public void save() throws IOException {
+        synchronized (this) { // Folia - region threading
         this.removeExpired(); // Paper - remove expired values before saving
         JsonArray jsonArray = new JsonArray();
         this.map.values().stream().map(storedEntry -> Util.make(new JsonObject(), storedEntry::serialize)).forEach(jsonArray::add);
@@ -104,9 +_,11 @@
         try (BufferedWriter writer = Files.newWriter(this.file, StandardCharsets.UTF_8)) {
             GSON.toJson(jsonArray, GSON.newJsonWriter(writer));
         }
+        } // Folia - region threading
     }
 
     public void load() throws IOException {
+        synchronized (this) { // Folia - region threading
         if (this.file.exists()) {
             try (BufferedReader reader = Files.newReader(this.file, StandardCharsets.UTF_8)) {
                 this.map.clear();
@@ -131,5 +_,6 @@
             }
             // Spigot end
         }
+        } // Folia - region threading
     }
 }
