TempFile creation fails with "read-only file system" error

I configured .gitlab-ci.yml on a project to upload artifacts. The zipping of artifacts work fine but gitlab-workhorse fails with the error below when trying to create a temp file.

Now this doesn't seem to be a problem with the file system, as I can open a shell with the gitlab user and create a file in the same location where workhorse fails. Im out of clues...

Feb 28 20:41:19 hal gitlab-workhorse[581]: 2016/02/28 20:41:19 error: handleFileUploads: extract files from multipart: create tempfile: open /usr/share/webapps/gitlab/shared/artifacts/tmp/uploads/upload_069648221: read-only file system Feb 28 20:41:19 hal gitlab-workhorse[581]: 192.168.1.1 127.0.0.1:48574 - - [2016-02-28 20:41:19.488073 +0000 GMT] "POST /ci/api/v1/builds/11/artifacts HTTP/1.1" 500 22 "" "Go-http-client/1.1" 0.018189