We have encountered the same problem raised by Vinay Gupta - Multiple graders cannot use rubric in SpeedGrader. As Ken Black explained in his response, this change is the result of a an update deployed in the Dec 9, 2017 Production Release. I understand and have also encountered the bug that this release was intended to fix.
The problem is that graders are no longer able to edit the rubric scores once the rubric has been used and saved by another grader. Previously, this functionality was available and was used as part of a grading workflow that involved assessment input from TAs.
Based on feedback from our faculty and comments from Matthew Weathers, Tasha Biesinger, Melinda Kraft, the common use case seems to be this: teachers sometimes delegate certain aspects of the assessment to TAs (e.g., formatting, length, etc.). This allows the teacher to focus on the content, cogency, and scholarship of the submission. TAs would use the rubric in SpeedGrader to enter scores for some of the criteria, and teachers would then complete the assessment of the other criteria.
To put it another way, we're looking for functionality to accommodate collaborative grading using a rubric in SpeedGrader (editing and/or appending scores to the same assessment rubric).
Right now, when a teacher clicks "View Rubric," they are only presented with a blank rubric. In other words, the only option right now is for the teacher to complete a new assessment. Adding options/functionality to "Edit Current Assessment" or "Create New Assessment" would accommodate this use case. The "Edit Current Assessment" option would load the existing scores, allowing the teacher to edit and/or append scores and finalize the assessment of the submission. The "New Assessment" option would load a blank rubric.