The backup/restore feature is still in a preview mode and officially supports only 2 GB of data. From the error message you posted ("backup is currenly in progress") it seems you probably hit a bug which was there and was fixed last week (the result of that bug was that there were some lingering backups which blocked subsequent backups).
Please try it again, you should be able to invoke it now. If you find another error message in operational logs, feel free to post it here (just leave the RequestId in it unscrambled - we can correlate using that) and we can take a look.
However, as I mentioned in the beginning, more than 2 GBs are not fully supported yet (you might not be able to do e.g. roundtrip with your data - backup and then restore).
Thanks, Petr