507 Insufficient Storage
The disk ran out — said with its own code, courtesy of WebDAV.
From RFC 4918: the server understood the request but has nowhere to write the result — full partition, user quota burst. A 500 would say "something went wrong"; 507 says exactly what.
It is the code Nextcloud and its kin return when an upload does not fit the quota. Outside WebDAV it appears on loan in storage APIs that preferred its precision over the generic 500.
HTTP/1.1 507 Insufficient Storage
Content-Type: application/xml; charset="utf-8"
Content-Length: 193