I had a track session that was split into intervals, so after each I reset the garmin essentially making 6 different sessions that each uploaded when I got home (Garmin Forerunner 610).
I have downloaded each of the activities from my activities list as .GPX files and a quick check they all look like valid XML.
First I merged the files into one (details of this are unimportant) and imported the file to get an "error occurred" message.
Thinking that is was a problem with the merging I validated the GPX file, but still no luck, so I thought I'd try something else.
I uploaded one of the _unaltered) .gpx files I got straight out of the activities list and got the same error.
My guess is that there's an actual bug/problem with the GPX importer that needs investigating, or at least the error should state whether it's a problem with my upload or a problem at the service end.
Also, as a feature request I'm sure already exists on here, a "merge activity" function through the site would make my day! :)