Lightroom Classic 15.4.1 cannot create catalog backups directly on SMB NAS shares (backup fails during compression)

Title: Lightroom Classic 15.4.1 cannot create catalog backups directly on SMB NAS shares (backup fails during compression)

Product:
Adobe Lightroom Classic 15.4.1

Platform:
macOS

Summary:
After updating to Lightroom Classic 15.4.1, catalog backups can no longer be created directly on an SMB-mounted NAS share. The backup process fails during the compression stage. Creating the same backup on a local SSD works correctly.

Environment:

• Lightroom Classic 15.4.1

• macOS

• Catalog stored on internal SSD

• Preview files stored locally

• Original photo files stored on NAS

• Backup destination: SMB-mounted NAS share

• NAS accessible and writable from Finder

Steps to Reproduce:

1. Open Lightroom Classic 15.4.1.

2. Use a catalog stored on a local SSD.

3. Configure Catalog Settings > Backups to save backups to an SMB-mounted NAS share.

4. Quit Lightroom and allow the catalog backup to run.

5. Lightroom starts the backup process and reaches the compression stage.

6. Backup fails with the message:

   "Lightroom could not back up the catalog. Check folder permissions and make sure sufficient space is available on both the backup drive and the drive containing the catalog."

Expected Result:

• Lightroom successfully creates a compressed catalog backup (.lrcat.zip) on the NAS share.

Actual Result:

• Backup fails during or immediately after compression.

• No valid .zip backup is created.

• Lightroom reports zero valid backups in Catalog Settings.

• Previously failed backup folders are not recognized as valid backups.

Troubleshooting Performed:

1. Verified sufficient free space on NAS and local SSD.

2. Verified read/write permissions on the NAS share.

3. Lightroom can create folders on the NAS.

4. Lightroom can write files to the NAS.

5. Finder can create, copy, and manage files on the NAS normally.

6. Creating a backup to a local SSD works correctly.

7. Lightroom successfully creates a valid catalog backup ZIP on the local SSD.

8. Manually copying that ZIP file to the NAS works without any issues.

9. After copying the ZIP to the NAS manually, Lightroom immediately recognizes it as a valid backup and displays it correctly in Catalog Settings.

Important Observation:

The problem appears to occur only when Lightroom itself attempts to create the ZIP file directly on the SMB share.

The following workflow succeeds:

• Create backup locally → OK

• Copy resulting ZIP file to NAS → OK

• Lightroom recognizes copied backup on NAS → OK

The following workflow fails:

• Create backup directly on NAS → FAIL

This suggests a problem in Lightroom's backup/compression routine when writing directly to SMB network volumes.

Additional Notes:

The issue started immediately after updating to Lightroom Classic 15.4.1. No changes were made to:

• NAS configuration

• SMB settings

• User permissions

• Folder structure

• macOS settings

The same NAS backup workflow worked correctly before the Lightroom update.