diff --git a/src-tauri/src/services/webdav_sync.rs b/src-tauri/src/services/webdav_sync.rs
index 7228c80d5..b7309b697 100644
--- a/src-tauri/src/services/webdav_sync.rs
+++ b/src-tauri/src/services/webdav_sync.rs
@@ -31,6 +31,12 @@ pub(crate) fn sync_mutex() -> &'static tokio::sync::Mutex<()> {
 
 pub(crate) mod archive;
 
+/// Content type for the manifest PUT. Some WebDAV servers (e.g. CSTCloud
+/// Data Capsule) reject multi-line bodies sent as `application/json` with a
+/// size-mismatch 500, while accepting the same bytes as a generic binary.
+/// The bytes are unchanged, so readers of any version are unaffected.
+const MANIFEST_CONTENT_TYPE: &str = "application/octet-stream";
+
 struct RemoteSnapshot {
     layout: RemoteLayout,
     manifest: SyncManifest,
@@ -73,7 +79,7 @@ pub async fn upload(
         &manifest_url,
         &auth,
         snapshot.manifest_bytes,
-        "application/json",
+        MANIFEST_CONTENT_TYPE,
     )
     .await?;