ImageOnMap: v4.1.2
Minecraft: 1.16.5
Language: Japanese

Config.yml:

# Plugin language. Empty: system language.
# Available: en-US (default, fallback), fr-FR, ru-RU, de-DE, zh-CN, ja-JP.
lang: <This bug occurs in both ja-JP and en-US.>

I changed the language to zh-CN and it has the same error!

I don't read chinese nor japanese, those characters are useless or are they replacing good one? I don't have this issue in en-US . It may be linked to an encoding issue in your server starting script. You had to specify the encoding with the argument -Dfile.encoding=UTF-8 in your start.bat or start.sh script. Do you use an alternate encoding for chinese or japanese ?

those characters are useless or are they replacing good one?

They are useless.

Do you use an alternate encoding for chinese or japanese?

I don't know what encoding is set by default.

My start.bat:

@echo off
java -Xms1G -Xmx1G -jar spigot-1.16.5.jar nogui
pause

I added the UTF-8 argument, and this bug was fixed.

Nice! I will look this if it's solved.