You are correct: The "Gathering Data Over the Web" topic mistakenly refers to encrypted data files having and *.inq extension and plain-text files having a *.dat extension. Those used to be the extensions up to and including Inquisit version 3.
Under Inquisit 4 and above the respective file extensions are *.iqdatc and *.iqdat, respectively. (The language reference for the /encrypt attribute has this correct: "Encrypted files are saved with the "*.iqdatc" file extension, and plain text files are saved with the ".iqdat" file extension.")
As for the decryption failure: This is definitely a nasty bug. Thank you for pointing it out.