Hi Phil,
I wrote the ticket about 3 months ago, you can review it here: http://trac.roundcube.net/trac.cgi/ticket/1483926
I seems they're blaming a blank space at the end of one of the php pages, I'll try and take a look at mine later, but if you have time, see if you can find it.
I think I found the problem - I've updated the ticket you kindly posted with information on the fix.
Yours, Morten