From the log file, it looks as if "listallinfo" failed. Cantata uses this 
command to get the music listing from mpd. Do you have this disabled in your 
MPD setup? (not sure if this can be done though!)

What happens if you run the following from a commandline:

1. Connect to to MPD via telnet:
    telnet localhost 6600

2. In the telnet window, type
    listallinfo

3. To quit telnet; press 'Ctrl' and  ']' together - then type 'quit' at the 
'telnet>' prompt.

Did the listallinfo work above?

-------------------

You mention KWallet - is your MPD password protected? With the trunk version of 
Cantata, you can disable the KWallet usage (pass -DENABLE_KWALLET=OFF to cmake)

Source: r3580 cannot connect with mpd 0.18.1 – “Failed to send command to “Default” (localhost:6,600) – not connected” · Issue #325 · CDrummond/cantata