If the ZIP file cannot be opened, it'd mean that the file was corrupted somehow, either during the compression, encryption, attachment, or transmission process.  Or it could be that the encryption key was simply not correct.

If the problem is, indeed, that you simply cannot open the ZIP file, that has nothing to do with the conversion process and the best thing to do is to have the prior accountant repackage the ZIP file and test it for integrity before sending the new file to you.