MalformedJsonException: Malformed JSON at Line 1 Column 1

What the log looks like

com\.google\.gson\.stream\.MalformedJsonException: Use JsonReader\.setLenient\(true\) to accept malformed JSON at line 1 column 1 path \$

How to fix it

1. Open server.properties and check generator-settings= if DedicatedServerProperties or GeneratorSettings appears in the log; clear the value completely or replace legacy preset strings with valid JSON such as {}.
2. Check resource-pack-prompt= in server.properties if parseResourcePackPrompt appears in the log, and format plain text as a valid JSON text component (for example, {"text":"A resource pack is required."}) or leave it empty.
3. If the stack trace points to WorldCreator.createWorld or a multi-world plugin, update that plugin for your Minecraft version and remove any pre-1.16 flat-world preset strings (like 2;0;1;) from its configuration.
4. Check usercache.json, ops.json, whitelist.json, banned-players.json, and recent mod config .json files in a text editor or JSON validator; if any file contains unreadable characters or incomplete text, delete or restore it from a backup so the game can regenerate a clean copy.

Check your own log