There isn't much information about this error if you look for it online. All you can find is a KB article from Dell about the firmware.
However, firmware is not the only reason why this error can appear.
This error also appears when the SMA is out of space and the SMA is configured to use radius for authentication.
There is simply not enough space to create the new user, especially when the users gets deleted after their session.
To fix this error, simply increase the storage on the appliance and it will auto-configure the new size by itself.
You can check if it's really a space issue by checking the httpd logs on the SMA.