Unknown plural rule “(n !”; without JavaScript engine available

[knniDE]

[00:08:35 WARN]: [ImageOnMap] Unknown plural rule “(n !”; without JavaScript engine available, well fallback to English pluralization rules. If you want your languages plural rules supported without JavaScript engine, please open an issue with your language and its plural rules at https://github.com/zDevelopers/QuartzLib/issues.

Paper version git-Paper-81 (MC: 1.18.1)
ImageOnMap version 4.2.1

java version "17.0.1" 2021-10-19 LTS
Java(TM) SE Runtime Environment (build 17.0.1+12-LTS-39)
Java HotSpot(TM) 64-Bit Server VM (build 17.0.1+12-LTS-39, mixed mode, sharing)

additional:
de_DE translations dont work in the GUI
the warning appears when you want to generate a map. (which also works)

[Vlammar]

Hello, can you make sure that the line "Plural-Forms: nplurals=2; plural=(n != 1);\n" is correctly set in your PO file?
For reference here is the PO file used for the german translation https://github.com/zDevelopers/ImageOnMap/blob/cc58b1d3200f63ceb514ecc10302315950f9a9a6/src/main/resources/i18n/de_DE.po

[knniDE]

I have not changed anything in the ImageOnMap-4.2.1.jar. Also I have not swapped out the translations. In the plugins folder is only the config.yml and images and maps folder.
I have nevertheless opened the jar with winrar and looked at the mentioned de_DE.po. There the line "Plural-Forms: nplurals=2; plural=(n != 1);\n" is correct.