There is one to many relationship between repositories and snapshots and you seemed to be conflating the two. One repository can contain multiple snapshots and you register repository while creating and restoring snapshots. Please, check documentation and fully understand the difference and relationship between them before continue.
The error that you are getting can indicate several things. It can indicate that some nodes don't have access to the directory
/shared/appdata/sls/sls.elasticsearch/backups and timeout trying to access it or that the cluster is in pretty bad shape and cannot process the request within 30 seconds.
Could you verify that the directory
/shared/appdata/sls/sls.elasticsearch/backups is mounted on all nodes and have the same access rights and that elasticsearch is running under the user with the same id on all nodes.
Could you also check that nothing else is going on on the cluster by running
Do you see any error message in the log file on the master node?