I can tell you where it comes from:

If the file has been downloaded and is complete, sometimes the program doesn't recognize it correctly, tries to download more, but there is nothing more to download... I had that more or less every day now and I solved the situation by renaming the RAR file from ...RAR.PART into ...RAR

And guess what... it unpacks the archive in a totally normal way

I guess that this is just a program flaw, nothing else

Judith