I'm sure it's a simple mistake and we'll all feel dumb for having asked, but an answer the first time would probably have prevented the duplicated work.
@Marcus Bertrand - telling everyone to email firstname.lastname@example.org and setting the issue as "invalid" will not solve the problem for the next person. I realize that http://answers.atlassian.com/ may be the more appropriate place to post this question but the site does not seem to be indexed by google very well as a search like "answers atlassian bitbucket conq: invalid command syntax" only yields the 3 links I listed.
The problem seems to be on bitbuckets side, and had something to do with the ssh server. The quickest solution is to switch to HTTPS:
As @Stu Campbell points out, this was an issue with some clients having an incorrect default configuration for the git-receive-pack command. We've changed our system to workaround this client issue so this issue should be resolved.
I had same error. Mine was caused by invalid remote URL. I added the remote using SourceTree (this was an initial push) by copying from bit bucket instructions. What got put in the config file was the URL twice with \n after each. So in my case was a SourceTree bug