You could try doing a manual upload on Garmin Connect.
The Forerunner should appear as a drive on your computer when you plug it in. So open that, then the folder Garmin\Activities. All of your activities should be there, as FIT files, each named with the date.
So you can copy the files off, and do a manual upload to Garmin Connect.
You could delete some of the activities off the Forerunner (after taking a backup copy if you want).
If you have loads of activities stored, it may be running out of memory, and could cause problems with using the Garmin Communicator plugin for uploads.