This indicates that the internal Git repository that stores DSS configuration versions is corrupted, probably following a sudden power loss (this can happen if you are using the virtualbox/vmware DSS and shut it down brutally).
To fix this, you need to log in to your DSS machine (if you are using virtualbox/vmware, the instructions appear in the banner).
* Note the project key of the project in which you have a problem
* Go to the DSS data directory, and then run: ./bin/dss stop
* rm -rf config/projects/THEPROJECTWITHTHEPROBLEM/.git
* ./bin/dss start
Alternatively, creating another project should work, if you don't have much work in your project for the moment