I've problems uploading data from my forerunner 310xt to Garmin Connect via Garmin express. I'm using 4.0.11, the latest version of express. I removed my device and tried to add it again. Garmin Express finds the device, but a 'connection to Garmin servers' error appears as I'm trying to sync with my Garmin connect account.
I had the same issue. Running the most recent firmware (4.50) and most recent Express (4.0.11). I went through all of the steps recommended by Garmin including:
1) Uninstall Garmin Express (remove app and ~/Library/Preferences ~/Library/Caches ~/Library/Application Support files)
2) Remove device from Garmin Connect
3) Check time/zone settings
4) Reinstall Garmin Express
And Finally
5) Master reset of 310xt -- SEE BELOW
None of items 1-4 resolved the problem on their own or in combination. Eventually I had to do a master reset of my 310xt to fix the problem. Note that doing a master reset will erase all settings on the device and delete all activities. You may be able to download your activities using the old Ant Agent (2.2.2) before performing the master reset. I was able to download and install the Ant Agent to get my activities onto Garmin Connect prior to the master reset. Why the old software still syncs the activities, while the new software does not is anyone's guess, but seems related to Garmin Express and possibly the recent update to 4.0.11 given the coincidence. The activities downloaded from the 310xt using the Ant Agent, and uploaded to Garmin Connect and Strava without any problem. The activities did not appear to be corrupt as Express suggested.
Would be interested to know what changes on Garmin's side precipitated this problem as it appears to be a somewhat common occurrence for users over the last few days. I'm keeping the Ant Agent disk image around just in case this happens again. Ant Agent can't be installed side-by-side with Express (to be precise, Ant Agent will be uninstalled when Express installs, but Ant Agent can be installed on top of Express), so this just adds an extra annoying step to recover activities, but it seems to be a good backup for times when the new and improved software and/or firmware doesn't work correctly.
