I have created a folder for storing the backups NUDF in NAS box with space of 1TB. The warrning occurs as in the screenshots.
Its is testing only am checking with 3 user even though this error occur
Edit the profile and check if any "quota" has been applied to the DUDF or not.
If not in the profile, check global settings as well from Tool - Options.
The error message is mentioning about the Desktop User Data folder which resides on the client machine and not the NAS. Check how much free space is present in the C:\ of the user machine.
To prevent this in the future, increase the free space, reduce the number of backup revisions for the DUDF and remove any quota if present.