Nothing like starting your day with an error like this:
CredisException: MISCONF Redis is configured to save RDB snapshots, but is currently not able to persist on disk.
Fortunately, this issue really has nothing to do with Redis, but only a pointer to another issue. The server resources.
First, if time permits and the server is accessible by SSH, grab some useful data from the CLI.
Both will give you an idea of what the problem may have been. Yes, ultimately your server may have run out of disk space, but why?
Did you run out of memory and required more use of a swap file?
After grabbing the data, reboot the server with a simple sudo reboot and then run the same commands again.
Did the drive space on the volume still show 100%? If not how much drive space is now available?
The redis log will show the cause of the error, but normally the root issue is lack of drive space or physical server memory.
To increase your EC2 volume you can review this KB article: Increase EC2 Volume
To increase your memory, just modify the AWS instance type and increase to a larger size.
Tip: Monitor both disk space and memory usage from the Area 51 dashboard.