Upload part size does not match
A multipart part body was not the expected size for that partIndex (from init’s totalBytes / chunkSize / totalParts).
Why this happens
Truncated upload, wrong part index, client/server chunk disagreement, or a corrupted temp part on disk. Live routes often say “Part size mismatch (expected N bytes).” Merge-time mismatches may surface as Could not finalize upload instead. The short catalog wording is the same family.
Diagnose and fix
1. Cancel and restart the upload so init and parts agree again.
2. API clients: send exact chunk sizes from init; last part may be shorter.
3. Check proxy buffering that might truncate PUT bodies; disk space under UPLOAD_DIR.
4. Not a Users & limits toggle — fix the client/session, then retry.