Message, "Network IO Error"

Possible causes of the Network IO Error:

Most of the time this error is the result of lack of permissions on the Appx directory. Appx users must have full control within the Appx directory and all sub-folders within it. They also need to be able to run the Appx executable on the server with logon local rights.

We have seen this error caused by a spam filter. If any new spam/virus filters been added to your server in the past few months, could you turn them off briefly to test connectivity?

You might get this error if you're running a newer version of the client than the engine. Example, running Appx 4.2.a with the 5.0.1 client.