Could not save upload part
Chunked upload failed while saving one part to temporary storage on the app (or API) server.
Why this happens
Desktop and Member API chunked uploads send each part to /api/upload/part (or /api/v1/upload/part). The server streams the bytes into a temp file under the upload directory, then renames it. If that write fails — disk full, bad permissions, read-only mount, or a stream error — you get this 500 message.
A wrong part size is a different message (“Part size mismatch…”). This one means the part never landed cleanly on disk.
Diagnose and fix
1. Disk space on the volume that holds uploads (UPLOAD_DIR, default data/uploads under the app root). Free space or move UPLOAD_DIR to a larger disk.
2. Permissions — the app process user must own and write that directory.
3. Mods → Desktop uploader (toggle.m_z) must be on for browser chunked parts; Member API upload mod for API parts.
4. Users & limits → Uploads enabled for the member’s tier (toggle.ul2_upl_on_*).
5. Check reverse-proxy body size / timeouts (Nginx client_max_body_size vs Upload hard cap). Ask the member to retry a small file first.