507 Insufficient Storage
Meaning
The server is unable to store the representation needed to complete the request. Often related to WebDAV operations, indicating that the server or resource does not have enough storage for the request.
When to Use
When a server (often in the context of WebDAV or similar file storage operations) runs out of space or quota to fulfill a request that would modify or create a resource. For example, an upload or file creation cannot be completed because of storage limitations.
Example
A WebDAV service responding to a PUT request to upload a file might return 507 Insufficient Storage if the user's storage quota is exceeded or the disk is full, preventing the file from being saved.
When Not to Use
Do not use if the failure isn't due to storage. This is specifically about storage capacity problems. Other server errors should use different codes (500 for generic, 509 for bandwidth if you choose to signal that separately, etc.).
Source
RFC 4918 (WebDAV)