No, it's not a limitation of the demo version, it's most likely because the GPX file has too many points - depending on where you upload to there are different limitations.
The active log is 10k, but you'd need to clear it first, and make sure it doesn't get overwritten when you're out there collecting a tracklog yourself.
Best place for uploading tracks is saved tracks. There are 20 and are 500 points each. Right click on your track and choose simplifiy/split/splint. Then use the split feature to cut it into 500 point chunks. As long as it was 10k or less to begin with, the file should then all fit into the saved tracks area.
Hope that helps.
