[doot]

Thanks for the script and debug support ErlendSB! All fixed now!

For others who experience the same problem - this was the issue... My music files are on an external/USB drive, which changes how the path appears in MusicIP for linux compared to how it appears in Ubuntu:

In Ubuntu (as shown in xbmc.log after the script failed)= /media/My Music/Album...
In MusicIP (as shown by viewing "song information" in MusicIP)= /media/My Music/./Album...

As suggested by ErlendSB, I simply amended the script to suit my system. In this case, the added "/./" in the file path for MusicIP. The line I amended is as follows:

currentlyPlaying = currentlyPlaying.replace("/My Music/","/My+Music/./").replace(" ","+")

I tested the need for the extra "/./" in the path by doing a direct call via web browser:
http:// localhost:XXXX/api/mix?song=/media/My+Music/./Album...&size=12&sizeType=tracks&content=text

(Where XXXX is my local api setting; and /Album... was the full path to the file I wanted to create a mix from).

For a while there I was afraid I'd have to move back to Windows to make use of MusicIP. This script, combined with XBMC, is better than the setup I had previously under Windows. And free I tells ya...