We never recommend to import more than 30 files (depending on the file format).
The "0" error happens mostly if there was an communcation error during the upload.
If you want we can import those files for you. Just send us a mail to email@example.com
and add the activties attached in dropbox/google drive. Don't forget to mention your username.
Don't forget. If you use Garmin or Polar: We offer a full import tool (See "Automatic Sync" -> Checkout our one-time full import tool for Garmin Connect). That should import all your activities by Garmin. This can take some hours.