Thanks for the hint. It works with 64bit but maybe also contains 32bit elements in the .so file that are not accepted by Google's Playstore (probably in the path UltimateArchiver.jar\lib\x86\libp7zip.so).
Not sure what the problem is? Maybe I have not understood the problem but in the readme file,
@Informatix says that you can add missing so-files from the native add-on folder he provided to the lib folder present in the jar-library.
In the native add-on folder :
you can find the arm64-v8a folder.
You should be able to unzip the jar file and add the arm64-v8a-folder (and its so-lib) and then you zip the file again (giving it the extension of .jar) and use this one. You can do the same with the x86_64 folder if you really want to support the x86 architecture/platform although hardly any devices uses it these days.
Edit: I missed
@JackKirk post. Well, he said what I wanted to say as well...