One of your activities (or courses) is corrupted (had the same problem). Found the solution in another forum (can't remember wich one...)
1) If you can find out wich one (e.g. by syncing them on batches to garmin web dashboard uploader), delete it from your watch.
2) After then remove your watch from Garmin Express
3) delete the device folder from your watch from your pc - found in "C:\ProgramData\Garmin\CoreService\Devices\xxx"
4) add your watch again in garmin express and sync it