If you are seeing the error "Malformed locale name: UTF8" when trying to generate Java documentation using IntelliJ IDEA, it means that there is an issue with the encoding of the Java source files that you are trying to document.
To fix this error, you need to make sure that the Java source files are saved with the correct encoding. In IntelliJ IDEA, you can check the encoding of a file by going to File > File Encoding.
If the encoding of the file is set to "UTF8", you should change it to "UTF-8". The correct spelling of the encoding is "UTF-8", with a hyphen, rather than "UTF8", which is incorrect.
Once you have corrected the encoding of the Java source files, try generating the Java documentation again. This should fix the "Malformed locale name: UTF8" error.
For more information on encoding in IntelliJ IDEA, you can refer to the IntelliJ IDEA documentation.