In T212428#6669851, @Geertivp wrote:

I got a similar error halfway running update.php when upgrading from 1.31 to 1.35. It seems that the slots table was created in 1.31.10 (minor version) and that the populateContentTables.php script normally should have been executed automatically by the update.php script. Luckily by running the populateContentTables.php manually I could run update.php a 2nd time to execute the rest of the upgrade (without requiring to restore the database to the initial backup). Here is the log tail from the 1st run of update.php

Modifying table site_stats ...done.^M
Populating ar_rev_id.^M
Populating ar_rev_id...^M
MediaWiki\Revision\RevisionAccessException from line 1296 of /data/html/mediawiki-1.35.0/includes/Revision/RevisionStore.php: Main slot of revision not found in database. See T212428.^M