Thanks, I had also found that Issue and was keeping it aside as a last resort option. After all, messing with stuff directly through the Rails Console is a bit dangerous. Your message gave me the confidence to try it myself 
For anyone else that encounters this, here is how I solved it. Bear in mind that there may be other causes for this or similar errors (in my case it seemed to really only be that the repository_read_only flag was already set, for whatever reason). Proceed at your own risk.
If you don’t already know which repositories/projects are affected, you should see a failed Sidekiq job for each under Admin Area > Monitoring > Background Jobs > Dead. The Repository storage Rake tasks can also come in handy to confirm the no. and list the projects stuck in legacy storage.
- Start a Rails console session
- Find the project by name:
p = Project.find_by_name("<project-name>") - Confirm that the repository is in fact read-only:
p.repository_read_only - Unset the repository_read_only flag:
p.update!(repository_read_only:nil) - Retry the corresponding Sidekiq job in the Admin Area (for me it would succeed almost instantly - the dead job count decreased and I never saw a running job)
- Rinse and repeat from step 2.