I tried to compress a set of files via batch processing. Some of these files were currently opened by another process, so I encountered this problem.
7zip works as designed to skip those files, because an opened file could be uncompleted in the moment where we are trying to read it for compression, so its best practice to skip them.
7z a -ssw archivename file_to_be_archived
or for older 7z Versions
7z a -x!TP archivename file_to_be_archived