Restore fails when downgrading WP version

Hi – I’ve gotten myself into a bit of a mess, with the free version of UpdraftPlus.

I created a complete UpdraftPlus backup. I then tried to update a site that had not been updated for a couple of years (WP 6.0.2, similar vintage plugins & themes). I first updated to current verison of WP (6.8.2). This was a mistake I think – probably should have started with updating the plugins & theme first, then the WP update.

Result: critical error. Several plugins – including UpdraftPlus – not loading due to the following error (I have access to ftp, error_log, etc.):

[07-Aug-2025 05:40:05 UTC] PHP Notice: Function _load_textdomain_just_in_time was called <strong>incorrectly</strong>. Translation loading for the <code>updraftplus</code> domain was triggered too early. This is usually an indicator for some code in the plugin or theme running too early. Translations should be loaded at the <code>init</code> action or later. Please see <a href=”https://developer.ww.wp.xz.cn/advanced-administration/debug/debug-wordpress/”>Debugging in WordPress</a> for more information. (This message was added in version 6.7.0.) in /home/fmydfhmqvdb1/public_html/wp-includes/functions.php on line 6121

So… my thought was: first restore the backup and start over. I managed to get WP up and running on 6.8.2, by disabling the offending plugins & theme, then updating UpdraftPlus, and running Restore.

UpdraftPlus restore then failed (some errors below):

0000.085 (R) [warning] Error: Existing unremoved folders from a previous restore exist (please use the “Delete old folders” button to delete them before trying again): /home/fmydfhmqvdb1/public_html/wp-content/updraft/plugins-old 0000.090 (R) [notice] Restore failed…

-also-

0014.381 () Unzip progress: 3 out of 3 files (28 B, index.php) 0014.382 () Cleaning up rubbish…
0014.382 () Error: Failed to delete working directory after restoring. (/home/fmydfhmqvdb1/public_html/wp-content/upgrade/dd3ece36)
0014.398 () PHP event: code E_USER_WARNING: The expected file /home/fmydfhmqvdb1/public_html/wp-content/plugins/updraftplus/vendor/brumann/polyfill-unserialize/src/DisallowedClassesSubstitutor.php is missing from your UpdraftPlus installation. Most likely, WordPress did not correctly unpack the plugin when installing it. You should de-install and then re-install the plugin (your settings and data will be retained). (line 97, wp-content/plugins/updraftplus/updraftplus.php) 0014.398 () PHP event: code E_WARNING: require_once(/home/fmydfhmqvdb1/public_html/wp-content/plugins/updraftplus/vendor/brumann/polyfill-unserialize/src/DisallowedClassesSubstitutor.php): failed to open stream: No such file or directory (line 107, wp-content/plugins/updraftplus/updraftplus.php)

So I’m kind of unsure about the way forward at this point. Any insights of course appreciated.

I didn’t realize the free version of UpdraftPlus doesn’t backup wp-admin & wp-Include files (& root files I assume). So it seems that might be why I’m unable (using UpdraftPlus plugin in WP 6.8.2) to restore the site, using the backups made with the free version (in WP 6.0.2) to WP 6.0.2.

If I’m on the right track here – first get the site back to the restored working site – running on WP6.0.2 – then proceed with a more carefully considered update strategy to WP 6.8.2 – what might be a good way to get there? I’ve downloaded a WP 6.0.2 installation zip file from the ww.wp.xz.cn release archive, so I have the release versions of the wp-admin & wp-include directories. I am able to use ftp & zip to restore the file system… just not sure what all needs to be done – in what order. I assume the db update is done last – and I am able to do that manually.

Thanks.