This error occurs if the unzipping process of the downloaded zip file (bibisco-win32-x64-2.3.1-SE.zip or bibisco-win32-x64-2.3.1-CE.zip) did not complete successfully.
In order to fix it, it's necessary to unzip the downloaded zip file again and wait for it to finish before running bibisco.exe.
A great tool for unzipping files, an alternative to the default Windows extractor, is 7-zip. 7-zip is free and open source.