# Some files could not be synced. See below for errors

**URL:** <https://help.nextcloud.com/t/some-files-could-not-be-synced-see-below-for-errors/247610>\
**Category:** 💻 Desktop\
**Tags:** desktop-client, mac, nc33, aio\
**Created:** [August 2, 2026, 9:50pm UTC](https://help.nextcloud.com/t/some-files-could-not-be-synced-see-below-for-errors/247610 "2026-08-02T21:50:15Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![apastuszak](https://help.nextcloud.com/letter_avatar/apastuszak/32/5_5575768a8748004e209b776fc1b2916d.png) [@apastuszak](https://help.nextcloud.com/u/apastuszak)\
**Post date:** [August 2, 2026, 9:50pm UTC](https://help.nextcloud.com/t/some-files-could-not-be-synced-see-below-for-errors/247610/1 "2026-08-02T21:50:16Z")

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I’ve decided to move Nextcloud from the tarball install I have been using to Nextcloud AIO running in Docker.

I have both running at the same time, and I am moving my files from one server to the other using my Mac, version 34.0 of the Mac Desktop client, and Finder. I am using File Provider.

Both versions of NextCloud are running 33.0.6.

To migrate files, I simply right-click to download locally, and then I drag them into another Finder window for the AIO instance. This deletes the files from my old Nextcloud instance and copies them onto the new one.

Well, after spending an hour doing this last night, now both servers show a red X in the client with the message “Some files could not be synced. See below for errors.” When I look at the list of files that were synced below, I don’t have any errors.

I see one error in the logs:

`DbalExceptionAn exception occurred while executing a query: SQLSTATE[23505]: Unique violation: 7 ERROR: duplicate key value violates unique constraint “previews_file_uniq_idx” DETAIL: Key (file_id, width, height, mimetype_id, cropped, version_id)=(189, 1024, 1024, 12, f, -1) already exists.`

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<div class="post-metadata">

**Author:** ![tflidd](https://help.nextcloud.com/letter_avatar/tflidd/32/5_5575768a8748004e209b776fc1b2916d.png) [@tflidd](https://help.nextcloud.com/u/tflidd)\
**Post date:** [August 3, 2026, 11:48am UTC](https://help.nextcloud.com/t/some-files-could-not-be-synced-see-below-for-errors/247610/2 "2026-08-03T11:48:55Z")

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It is the previews, I don’t know if the client triggers the generation. If it is just for the copying, you could disable the preview for the time:

> **[enable\_previews - Previews configuration — Nextcloud 34 Administration Manual](https://docs.nextcloud.com/server/stable/admin_manual/configuration_files/previews_configuration.html#disabling-previews)**
>
> Please notice that the Nextcloud preview system comes already with sensible defaults, and therefore it is usually unnecessary to adjust those configuration values.

and then uses the previewgenerateor in the end after enabling, to generate all the preview with a cronjob.

> [@apastuszak](#):
>
> To migrate files, I simply right-click to download locally, and then I drag them into another Finder window for the AIO instance.

I’d rather copy them, to keep one instance untouched, so you do not potentially end up with two broken setups so you have to recover from a backup.