failed to lock

With git-1.7, I regularly get this when pushing to a non-bare repo:

remote: error: failed to lock refs/remotes/origin/master
...
 ! [remote rejected] origin/master -> origin/master (failed to lock)
error: failed to push some refs to 'ssh://remote/repo'

I see this pushing from a Mac, to either a Mac or to Cygwin. I have seen it on various versions of the 1.7 series and possibly before. In the instance above:

local git version 1.7.0.3.436.g2b878 remote git version 1.7.0.3

In addition, the ref is (apparently) updated just fine. If I push a second time, no update occurs.

The problem is reproducible. Any idea where I should start debugging it?

j.