I went round in circles for a whole night, installing, uninstalling, trying different versions, reading every post about the updater not working properly... All to no avail.
Right throughout I gave all planeshift processes full access through my software firewall (Agnitum Outpost Pro 2009) and even tried disabling it. Still nothing so I carried on my merry dance.
Anyway - what I didn't do, until just now, was actually stop the whole firewall program from running (i.e. exit, rather than just 'disable firewall') and the updater worked perfectly.
Pretty obvious I know, but if I'd read this last night I'd have been much happier than I was.
Good luck anyone who's having trouble with it
